amd64 including:
- Add 32-bit wrappers for thr_new(), thr_suspend(), and the umtx system
calls.
- Add support to amd64 for constructing thread upcalls for 32-bit
processes.
- Leave %fs and %gs alone in the signal trampoline for 32-bit processes on
amd64.
- Add 'casuword32()' to amd64 and ia64.
Tested by: emaste
9d52499b7d