Fix makecontext() on MIPS O32.

The GP register can be clobbered by the callback, so save it in S1
while invoking the callback function.

While here, add a comment expounding on the treatment of GP for the
various ABIs and the assumptions made.

Reviewed by: jmallett (earlier version)
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D14179