[gdb/testsuite] Rewrite valgrind-db-attach.exp to use vgdb

[gdb/testsuite] Rewrite valgrind-db-attach.exp to use vgdb

The valgrind option --db-attach has been deprecated in version 3.10.0, and
removed in version 3.11.0, so the valgrind-db-attach.exp testcase is
unsupported starting version 3.11.0.

Rewrite the test-case to use vgdb instead (making it supported starting
version 3.7.0).

Tested on x86_64-linux with and without --target_board=native-gdbserver.

2018-10-25  Tom de Vries  <tdevries@suse.de>

	* gdb.base/valgrind-db-attach.exp: Rewrite to use vgdb.