Implement getcontext(), setcontext(), makecontext(), and swapcontext().

This is a preliminary libc-only implementation. sigprocmask() is used
to mask signals during the context switch and to save a

Implement getcontext(), setcontext(), makecontext(), and swapcontext().

This is a preliminary libc-only implementation. sigprocmask() is used
to mask signals during the context switch and to save and restore signals.
FP registers are not currently saved or restored - that is, synchronous
use is intended.

Submitted-by: "Matt Emmerton" <matt@gsicomp.on.ca>
Finished-up-by: "Matthew Dillon" <dillon@backplane.com>

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