gdb: add gdbarch method to get execution context from core file

Add a new gdbarch method which can read the execution context from a
core file.  An execution context, for this commit, means the filename
of the executable used to generate the core file and the arguments
passed to the executable.

In later commits this will be extended further to include the
environment in which the executable was run, but this commit is
already pretty big, so I've split that part out into a later commit.

Initially this new gdbarch method is only implemented for Linux
targets, but a later commit will add FreeBSD support too.

Currently when GDB opens a core file, GDB reports the command and
arguments used to generate the core file.  For example:

  (gdb) core-file ./core.521524
  [New LWP 521524]
  Core was generated by `./gen-core abc def'.

However, this information comes from the psinfo structure in the core
file, and this struct only allows 80 characters for the command and
arguments combined.  If the command and arguments exceed this then
they are truncated.

Additionally, neither the executable nor the arguments are quoted in
the psinfo structure, so if, for example, the executable was named
'aaa bbb' (i.e. contains white space) and was run with the arguments
'ccc' and 'ddd', then when this core file was opened by GDB we'd see:

  (gdb) core-file ./core.521524
  [New LWP 521524]
  Core was generated by `./aaa bbb ccc ddd'.

It is impossible to know if 'bbb' is part of the executable filename,
or another argument.

However, the kernel places the executable command onto the user stack,
this is pointed to by the AT_EXECFN entry in the auxv vector.
Additionally, the inferior arguments are all available on the user
stack.  The new gdbarch method added in this commit extracts this
information from the user stack and allows GDB to access it.

The information on the stack is writable by the user, so a user
application can start up, edit the arguments, override the AT_EXECFN
string, and then dump core.  In this case GDB will report incorrect
information, however, it is worth noting that the psinfo structure is
also filled (by the kernel) by just copying information from the user
stack, so, if the user edits the on stack arguments, the values
reported in psinfo will change, so the new approach is no worse than
what we currently have.

The benefit of this approach is that GDB gets to report the full
executable name and all the arguments without the 80 character limit,
and GDB is aware which parts are the executable name, and which parts
are arguments, so we can, for example, style the executable name.

Another benefit is that, now we know all the arguments, we can poke
these into the inferior object.  This means that after loading a core
file a user can 'show args' to see the arguments used.  A user could
even transition from core file debugging to live inferior debugging
using, e.g. 'run', and GDB would restart the inferior with the correct
arguments.

Now the downside: finding the AT_EXECFN string is easy, the auxv entry
points directly too it.  However, finding the arguments is a little
trickier.  There's currently no easy way to get a direct pointer to
the arguments.  Instead, I've got a heuristic which I believe should
find the arguments in most cases.  The algorithm is laid out in
linux-tdep.c, I'll not repeat it here, but it's basically a search of
the user stack, starting from AT_EXECFN.

If the new heuristic fails then GDB just falls back to the old
approach, asking bfd to read the psinfo structure for us, which gives
the old 80 character limited answer.

For testing, I've run this series on (all GNU/Linux) x86-64. s390,
ppc64le, and the new test passes in each case.  I've done some very
basic testing on ARM which does things a little different than the
other architectures mentioned, see ARM specific notes in
linux_corefile_parse_exec_context_1 for details.

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