gdb: check for multiple matching build-id filesHEADmaster

Within the debug-file-directory GDB looks for the existence of a
.build-id directory.

Within the .build-id directory GDB looks for files with the form:

  .build-id/ff/4b4142d62b399499844924d53e33d4028380db.debug

which contain the debug information for the objfile with the build-id
ff4b4142d62b399499844924d53e33d4028380db.

There appear to be two strategies for populating the .build-id
directory.  Ubuntu takes the approach of placing the actual debug
information in this directory, so
4b4142d62b399499844924d53e33d4028380db.debug is an actual file
containing the debug information.

Fedora, RHEL, and SUSE take a slightly different approach, placing the
debug information elsewhere, and then creating symlinks in the
.build-id directory back to the original debug information file.  The
actual debug information is arranged in a mirror of the filesystem
within the debug directory, as an example, if the debug-file-directory
is /usr/lib/debug, then the debug information for /bin/foo can be
found in /usr/lib/debug/bin/foo.debug.

Where this gets interesting is that in some cases a package will
install a single binary with multiple names, in this case a single
binary will be install with either hard-links, or symlinks providing
the alternative names.

The debug information for these multiple binaries will then be placed
into the /usr/lib/debug/ tree, and again, links are created so a
single file can provide debug information for each of the names that
binary presents as.  An example file system might look like this (the
[link] could be symlinks, but are more likely hard-links):

  /bin/
    foo
    bar -> foo	[ HARD LINK ]
    baz -> foo	[ HARD LINK ]
  /usr/
    lib/
      debug/
        bin/
	  foo.debug
	  bar.debug -> foo.debug	[ HARD LINK ]
	  baz.debug -> foo.debug	[ HARD LINK ]

In the .build-id tree though we have a problem.  Do we have a single
entry that links to one of the .debug files?  This would work; a user
debugging any of the binaries will find the debug information based on
the build-id, and will get the correct information, after all the
.debug files are identical (same file linked together).  But there is
one problem with this approach.

Sometimes, for *reasons* it's possible that one or more the linked
binaries might get removed, along with its associated debug
information.  I'm honestly not 100% certain under what circumstances
this can happen, but what I observe is that sometime a single name for
a binary, and its corresponding .debug entry, can be missing.  If this
happens to be the entry that the .build-id link is pointing at, then
we have a problem.  The user can no longer find the debug information
based on the .build-id link.

The solution that Fedora, RHEL, & SUSE have adopted is to add multiple
entries in the .build-id tree, with each entry pointing to a different
name within the debug/ tree, a sequence number is added to the
build-id to distinguish the multiple entries.  Thus, we might end up
with a layout like this:

  /bin/
    foo
    bar -> foo	[ HARD LINK ]
    baz -> foo	[ HARD LINK ]
  /usr/
    lib/
      debug/
        bin/
	  foo.debug
	  bar.debug -> foo.debug	[ HARD LINK ]
	  baz.debug -> foo.debug	[ HARD LINK ]
      .build-id/
        a3/
          4b4142d62b399499844924d53e33d4028380db.debug -> ../../debug/bin/foo.debug	[ SYMLINK ]
          4b4142d62b399499844924d53e33d4028380db.1.debug -> ../../debug/bin/bar.debug	[ SYMLINK ]
          4b4142d62b399499844924d53e33d4028380db.2.debug -> ../../debug/bin/baz.debug	[ SYMLINK ]

With current master GDB, debug information will only ever be looked up
via the 4b4142d62b399499844924d53e33d4028380db.debug link.  But if
'foo' and its corresponding 'foo.debug' are ever removed, then master
GDB will fail to find the debug information.

Ubuntu seems to have a much better approach for debug information
handling; they place the debug information directly into the .build-id
tree, so there only ever needs to be a single entry for any one
build-id.  I wonder if/how they handle the case where multiple names
might share a single .debug file, if one of those names is then
uninstalled, how do they know the .debug file should be retained or
not ... but I assume that problem either doesn't exist or has been
solved.

Anyway, for a while Fedora has carried a patch that handles the
build-id sequence number logic.  What's presented here is inspired by
the Fedora patch, but has some changes to fix some issues.

I'm aware that this is a patch that applies to only some (probably a
minority) of distros.  However, the logic is contained to only a
single function in build-id.c, and isn't too complex, so I'm hoping
that there wont be too many objections.

For distros that don't have build-id sequence numbers there should be
no impact.  The sequence number approach still leaves the first file
without a sequence number, and this is the first file that GDB (after
this patch) checks for.  The new logic only kicks in if the
non-sequence numbered first file exists, but is a symlink to a non
existent file; in this case GDB checks for the sequence numbered files
instead.

Tests are included.

There is a small fix needed for gdb.base/sysroot-debug-lookup.exp,
after this commit GDB now treats a target: sysroot where the target
file system is local to GDB the same as if the sysroot had no target:
prefix.  The consequence of this is that GDB now resolves a symlink
back to the real filename in the sysroot-debug-lookup.exp test where
it didn't previously.  As this behaviour is inline with the case where
there is no target: prefix I think this is fine.

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