[gdb/breakpoints] Fix fullname.exp when run from symlink dir

I run into this error with gdb.base/fullname.exp:
...
(gdb) file /data/gdb_versions/devel/build/gdb/testsuite/outputs/\
  gdb.base/fullname/fullname
Reading symbols from /data/gdb_versions/devel/build/gdb/testsuite/outputs/\
  gdb.base/fullname/fullname...
(gdb) break /data/gdb_versions/devel/build/gdb/testsuite/\
  outputs/gdb.base/fullname/tmp-fullname.c:21
No source file named /data/gdb_versions/devel/build/gdb/testsuite/outputs/\
  gdb.base/fullname/tmp-fullname.c.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb) FAIL: gdb.base/fullname.exp: set breakpoint by full path before loading symbols - built relative
...

The FAIL is due to this comparison in iterate_over_some_symtabs failing:
...
481                   if (FILENAME_CMP (real_path, fullname) == 0)
(gdb) p real_path
$2 = 0x1a201f0 "/data/gdb_versions/devel/build/gdb/testsuite/outputs/\
  gdb.base/fullname/tmp-fullname.c"
(gdb) p fullname
$3 = 0x1a1de80 "/home/vries/gdb_versions/devel/build/gdb/testsuite/outputs/\
  gdb.base/fullname/tmp-fullname.c"
...

The difference in pathnames is due to having a symlink dir:
...
$ ls -la /home/vries/gdb_versions
lrwxrwxrwx 1 vries users 18 26 jun  2018 /home/vries/gdb_versions -> /data/gdb_versions
...
and the test passses when eliminating it:
...
$ ( cd $(pwd -P); make check RUNTESTFLAGS=gdb.base/fullname.exp )
...

The FAIL is a regression from commit a0c1ffed "Only compute realpath when
basenames_may_differ is set".  Before, find_and_open_source was returning a
real-path, resulting in variable 'fullname' being the same as varible
'real_path' in the comparison listed above.  But after, that's no longer the
case.

Fix the FAIL by applying gdb_realpath on the fullname variable before the
comparison.

Tested on x86_64-linux.

I wasn't able to write a test-case.  The FAIL starts at:
...
$ cd build/gdb
$ mv testsuite testsuite.bla
$ ln -s testsuite.bla testsuite
...
but already this doesn't trigger it anymore:
...
$ cd build/gdb/outputs
$ mv outputs outputs.bla
$ ln -s outputs.bla outputs
...

gdb/ChangeLog:

2019-10-23  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/24687
	* symtab.c (iterate_over_some_symtabs): Apply gdb_realpath on fullname.

Change-Id: I1ace62a234458781e958980f3b425edf1490df27