Rename some JL_HAVE_* constants to `JL_TASK_SWITCH_*` for clarity, and some related cleanup (#54911)
- **Replace JL_HAVE_UCONTEXT by JL_TASK_SWITCH_WINDOWS**
- **Untangle OpenBSD checks in julia_threads.h**
- **Rename JL_HAVE_UNW_CONTEXT -> JL_TASK_SWITCH_LIBUNWIND**
- **Rename JL_HAVE_ASM -> JL_TASK_SWITCH_ASM**
Whenever I try to understand the task switch code, I have to figure out
again when which of the various `JL_HAVE_*` constants are set, how they
interact, etc. -- and indeed recall that `JL_HAVE_*` controls the task
switching code in the first place.
So it seems sensible to me to rename these to indicate they control task
switching. This is in particular true for `JL_HAVE_UCONTEXT` which
was/is exclusively used on on Windows, despite the reference to
`ucontext`, a POSIX feature (albeit one removed from the standard in
POSIX.1-2008).
Of course it is still conceivable that in the future someone wants to
port Julia to a new platform and for some reason libunwind switching is
not workable, but the `ucontext` one is -- in that case it wouldn't be
too hard to add back a `JL_TASK_SWITCH_UCONTEXT`. But IMHO such a
hypothetical future is a good reason to confuse people trying to
understand the code today.
julia
ba139334 - Rename some JL_HAVE_* constants to `JL_TASK_SWITCH_*` for clarity, and some related cleanup (#54911)
