kselftest/arm64: Add a test for vfork() with GCS

kselftest/arm64: Add a test for vfork() with GCS

Ensure that we've got at least some coverage of the special cases around
vfork() by adding a test case in basic-gcs doing the same thing as the
plain fork() one - vfork(), do a few checks and then return to the parent.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250703-arm64-gcs-vfork-exit-v3-3-1e9a9d2ddbbe@kernel.org
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>