[gdb/testsuite] Handle supports_memtag in gdb.base/gdb-caching-proc.exp

[gdb/testsuite] Handle supports_memtag in gdb.base/gdb-caching-proc.exp

In test-case gdb.base/gdb-caching-proc.exp, we run all procs declared with
gdb_caching_proc.  Some of these require a gdb instance, some not.

We could just do a clean_restart every time, but that would amount to 44 gdb
restarts.  We try to minimize this by doing this only for the few procs that
need it, and hardcoding those in the test-case.

For those procs, we do a clean_restart, execute the proc, and then do a
gdb_exit, to make sure the gdb instance doesn't linger such that we detect
procs that need a gdb instance but are not listed in the test-case.

However, that doesn't work in the case of gnat_runtime_has_debug_info.  This
proc doesn't require a gdb instance because it starts its own.  But it doesn't
clean up the gdb instance, and since it's not listed, the test-case
doesn't clean up the gdb instance eiter.  Consequently, the proc
supports_memtag (which should be listed, but isn't) uses the gdb instance
started by gnat_runtime_has_debug_info rather than throwing an error.  Well,
unless gnat_runtime_has_debug_info fails before starting a gdb instance, in
which case we do run into the error.

Fix this by:
- doing gdb_exit unconditionally
- fixing the resulting error by adding supports_memtag in the test-case to
  the "needing gdb instance" list

Tested on x86_64-linux.