Mask out SRR1 bits that aren't exported to the MSR.

This appears to fix a strange condition with X on 32-bit PowerBooks I observed,
caused by one of these bits getting set in the mcontext, but not set in the
thread, which may be a symptom of another problem, more difficult to diagnose.
Since these bits aren't exported anyway, this change makes it more explicit that
the bits aren't MSR-related in SRR1.

MFC after:	3 weeks