MFC r317845-r317846

r317845:
Provide a freebsd32 implementation of sigqueue()

The previous misuse of sys_sigqueue() was sending random register or
stack garbage to 64-bit targets. The freebsd32 implementation preserves
the sival_int member of value when signaling a 64-bit process.

Document the mixed ABI implementation of union sigval and the
incompability of sival_ptr with pointer integrity schemes.

Reviewed by: kib, wblock
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10605

r317846:
Regen post r317845.

MFC with: r317845
Sponsored by: DARPA, AFRL