MFC r261095,r263464,r263752,r264189

r263464,r263752,r275189:

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 is 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.

r261095:

Fix 32-bit signal handling on ppc64.  This was broken when the
PSL_USERSTATIC macro was changed.  Since copying 64-bit srr1 into
32-bit srr1 drops the upper 32 bits, any bits set in the context were
dropped, meaning the context check fails.  Since 32-bit set_context()
can't change those bits anyway, copy the ones from the current context
(td->td_frame) before calling set_context().

Approved by:	re
Relnotes:	yes (Affects 10-stable, but not 10.0-release)