x86: make mxcsr_feature_mask static again

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: hpa@zytor.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>