[stdlib] Condition Mutex.swift on platform

[Azoy]

Mutex.swift was previously never conditional and always built, but the handle was conditioned based on platform. In a minimal toolchain, we don't have a platform to piggyback on, so we shouldn't build Mutex.swift either.

Resolves: rdar://129287049

[Azoy]

@swift-ci please smoke test

[Azoy]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with preset

[Azoy]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[stephentyrone]

Are we going to end up having to be able to check for availability? Should we put an unavailable placeholder in to give people a better error on other platforms? Also, do we need this on 6.0?

[Azoy]

It seems like we should be able to have an unavailable availability for Mutex on platforms that don't support it (we can compile a special version of Mutex on those platforms). Yes we do, I'm going to put up a PR with this and the Mutex changes to 6.0

[Azoy]

@swift-ci please smoke test

[Azoy]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset