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().

MFC after:	3 weeks