These should hopefully all be uncontroversial, minimal changes to deal
with progressing the build to completion on OpenBSD or addressing minor
portability issues. This is not the full set of changes to get a
successful build; other portability issues will be addressed in future
commits.
Most of this is just adding the relevant clauses to the ifdefs, but of
note in this commit:
* StdlibUnittest.swift: the default conditional in _getOSVersion assumes
an Apple platform, therefore the explicit conditional and the relevant
enums need filling out. The default conditional should be #error, but
we'll fix this in a different commit.
* tgmath.swift.gyb: inexplicably, OpenBSD is missing just lgammal_r.
Tests are updated correspondingly.
* ThreadLocalStorage.h: we use the pthread implementation, so it
seems we should typedef __swift_thread_key_t as pthread_key_t.
However, that's also a tweak for another commit.
Use `ThreadHandle(bitPattern:)` rather than `unsafeBitCast` for the
conversion of the `uintptr_t` to the `HANDLE`. Convert a variable from
`var` to `let` binding.
The recent changes to the UnsafemutablePointer prevented the "abuse" of
the type for casting. Switch to `unsafeBitCast`. This should repair
the Windows build.
This is in preparation to make the code here more target agnostic for
porting to the Windows threading primitives. This is used pretty
extensively in the tests, so disabling tests would lose a chunk of
coverage.
