libstdc++: Fix testsuite for skipping gdb tests on remote/non-native target

This fixes an incorrect invocation of gdb on remote targets where
DejaGNU would try to run host's gdb in remote target simulator.
gdb-test skips the testing when target is remote or non native but the
gdb version check function does not.

Suggested-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Luc Michel <lmichel@kalray.eu>

Co-authored-by: Marc Poulhies <mpoulhies@kalrayinc.com>

libstdc++-v3/ChangeLog:

	* testsuite/lib/gdb-test.exp (gdb_version_check)
	(gdb_version_check_xmethods): Only check the GDB version for
	local native targets.