[gdb/tdep] Fix use-after-free in arm_exidx_fill_cache

[gdb/tdep] Fix use-after-free in arm_exidx_fill_cache

On arm-linux the linaro CI occasionally reports:
...
 (gdb) up 10
 #4  0x0001b864 in pthread_join ()
 (gdb) FAIL: gdb.threads/staticthreads.exp: up 10
...
while this is expected:
...
 (gdb) up 10
 #3  0x00010568 in main (argc=1, argv=0xfffeede4) at staticthreads.c:76
 76          pthread_join (thread, NULL);
 (gdb) PASS: gdb.threads/staticthreads.exp: up 10
...

Thiago investigated the problem, and using valgrind found an invalid read in
arm_exidx_fill_cache.

The problem happens as follows:
- an objfile and corresponding per_bfd are allocated
- some memory is allocated in arm_exidx_new_objfile using
  objfile->objfile_obstack, for the "exception table entry cache".
- a symbol reread is triggered, and the objfile, including the
  objfile_obstack, is destroyed
- a new objfile is allocated, using the same per_bfd
- again arm_exidx_new_objfile is called, but since the same per_bfd is used,
  it doesn't allocate any new memory for the "exception table entry cache".
- the "exception table entry cache" is accessed by arm_exidx_fill_cache,
  and we have a use-after-free.

This is a regression since commit a2726d4ff80 ("[ARM] Store exception handling
information per-bfd instead of per-objfile"), which changed the "exception
table entry cache" from per-objfile to per-bfd, but failed to update the
obstack_alloc.

Fix this by using objfile->per_bfd->storage_obstack instead of
objfile->objfile_obstack.

I couldn't reproduce the FAIL myself, but Thiago confirmed that the patch
fixes it.

Tested on arm-linux.

Approved-By: Luis Machado <luis.machado@arm.com>
PR tdep/31254
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31254