x86/purgatory: disable KMSAN instrumentation

The stand-alone purgatory.ro does not contain the KMSAN runtime, therefore
it can't be built with KMSAN compiler instrumentation.

Link: https://lkml.kernel.org/r/20221024212144.2852069-2-glider@google.com
Link: https://github.com/google/kmsan/issues/89


Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>