the register values coming back from sigreturn(2).  Normally this wouldn't
matter because the 32 bit environment would truncate the upper 32 bits
and re-save the truncated values at the next trap.  However, if we got
a fast second signal and it was pending while we were returning from
sigreturn(2) in the signal trampoline, we'd never have had a chance to
truncate the bogus values in 32 bit mode, and the new sendsig would get
an EFAULT when trying to write to the bogus user stack address.