std: stop exposing anything having to do with ucontext_t

This type is useful for two things:

* Doing non-local control flow with ucontext.h functions.
* Inspecting machine state in a signal handler.

The first use case is not one we support; we no longer expose bindings to those
functions in the standard library. They're also deprecated in POSIX and, as a
result, not available in musl.

The second use case is valid, but is very poorly served by the standard library.
As evidenced by my changes to std.debug.cpu_context.signal_context_t, users will
be better served rolling their own ucontext_t and especially mcontext_t types
which fit their specific situation. Further, these types tend to evolve
frequently as architectures evolve, and the standard library has not done a good
job keeping up, or even providing them for all supported targets.

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