selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write

The test validates that the selector is not changed when a ptracer writes
the ptracee's GS base.

Originally-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200528201402.1708239-16-sashal@kernel.org