[gdb/testsuite] Fix gdb.dwarf2/gdb-index-nodebug.exp with cc-with-gdb-index

[gdb/testsuite] Fix gdb.dwarf2/gdb-index-nodebug.exp with cc-with-gdb-index

When running with target board cc-with-gdb-index, we run into:
...
(gdb) save gdb-index .^M
Error while writing index for `gdb-index-nodebug': \
  Cannot use an index to create the index^M
(gdb) FAIL: gdb.dwarf2/gdb-index-nodebug.exp: try to save gdb index
...

Fix this by detecting an already present index, and marking the test
unsupported.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-02  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/gdb-index-nodebug.exp: Mark unsupported when index
	already present.