x32: Signal-related system calls

x32 uses the 64-bit signal frame format, obviously, but there are some
structures which mixes that with pointers or sizeof(long) types, as
such we have to create a handful of system calls specific to x32.  By
and large these are a mixture of the 64-bit and the compat system
calls.

Originally-by: H. J. Lu <hjl.tools@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>