Fix x86oid OSX signal handling emulation assembly fragments.

Fix x86oid OSX signal handling emulation assembly fragments.

  * Both x86 and x86-64 signal_emulation_wrapper include a small
assembly fragment to simulate "sigreturn" by means of an invalid
instruction trap.  This fragment has to load two different pointers
into specific registers before the trap, but historically just told
the compiler to load them into registers and then moved them into
the correct registers, leading to the possibility of clobbering one
of the values.  Fixed, by informing the compiler to place them into
the correct registers to begin with.