I made a couple of decisions for this based on the fact that we don't expose the
signal_ucontext_t type outside of the file:
* Adding all the floating point and vector state to every ucontext_t and
mcontext_t variant was way, way too much work, especially when we don't even
use the stuff. So I deleted all that and kept only the bare minimum needed to
reach into general-purpose registers.
* There is no particularly compelling reason to stick to the naming and struct
nesting used in the system headers. So we can actually unify the access
patterns for almost all of these variants by taking some liberties here; as a
result, fromPosixSignalContext() is now much nicer to read and extend.
There were only a few dozen lines of common logic, and they frankly
introduced more complexity than they eliminated. Instead, let's accept
that the implementations of `SelfInfo` are all pretty different and want
to track different state. This probably fixes some synchronization and
memory bugs by simplifying a bunch of stuff. It also improves the DWARF
unwind cache, making it around twice as fast in a debug build with the
self-hosted x86_64 backend, because we no longer have to redundantly go
through the hashmap lookup logic to find the module. Unwinding on
Windows will also see a slight performance boost from this change,
because `RtlVirtualUnwind` does not need to know the module whatsoever,
so the old `SelfInfo` implementation was doing redundant work. Lastly,
this makes it even easier to implement `SelfInfo` on freestanding
targets; there is no longer a need to emulate a real module system,
since the user controls the whole implementation!
There are various other small refactors here in the `SelfInfo`
implementations as well as in the DWARF unwinding logic. This change
turned out to make a lot of stuff simpler!
