[gdb/testsuite] Prevent proc override in gdb-index.exp

[gdb/testsuite] Prevent proc override in gdb-index.exp

When running these two test-cases in this specific order we get:
...
$ make check 'RUNTESTFLAGS=gdb.dwarf2/gdb-index.exp \
    gdb.dwarf2/gdb-add-index-symlink.exp'
  ...
Running gdb.dwarf2/gdb-index.exp ...
Running gdb.dwarf2/gdb-add-index-symlink.exp ...
FAIL: gdb.dwarf2/gdb-add-index-symlink.exp: gdb-index file created
FAIL: gdb.dwarf2/gdb-add-index-symlink.exp: Unable to call \
  gdb-add-index with a symlink to a symfile
...

The problem is that gdb-index.exp introduces a proc add_gdb_index which
overrides the one in lib/gdb.exp and stays active after the test is done.
Consequently it's used in gdb-add-index-symlink.exp, which should use the one
from lib/gdb.exp.

Fix this by renaming proc add_gdb_index in gdb-index.exp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-05-27  Tom de Vries  <tdevries@suse.de>

	PR testsuite/27921
	* gdb.dwarf2/gdb-index.exp (add_gdb_index): Rename to ...
	(local_add_gdb_index): ... this.