[gdb/testsuite] Handle removed valgrind option --db-attach

[gdb/testsuite] Handle removed valgrind option --db-attach

When running valgrind-db-attach.exp with valgrind version 3.13.0, we get:
...
PASS: gdb.base/valgrind-db-attach.exp: spawn valgrind
valgrind: Unknown option: --db-attach=yes
valgrind: Use --help for more information or consult the user manual.
ERROR: Process no longer exists
UNRESOLVED: gdb.base/valgrind-db-attach.exp: valgrind started
...

The valgrind option --db-attach has been deprecated in version 3.10.0, and
removed in version 3.11.0.

Fix valgrind-db-attach.exp to replace the ERROR/UNRESOLVED with:
...
UNSUPPORTED: gdb.base/valgrind-db-attach.exp: valgrind started
...

Tested on x86_64-linux.

2018-10-24  Tom de Vries  <tdevries@suse.de>

	* gdb.base/valgrind-db-attach.exp: Handle removed support for
	--db-attach in valgrind.