Enable futex support in macOS 14.4+ through os_sync_wait_on_address

[hpnkv]

Hi! Starting with version 14.4, macOS SDK exposes a header that allows to operate with futexes: https://developer.apple.com/documentation/os/os_sync_wait_on_address.

I made a quick change, and it seems to work in my application: with the content of this PR, by defining BOOST_FIBERS_SPINLOCK_TTAS_FUTEX and an appropriate CMAKE_OSX_DEPLOYMENT_TARGET, I am able to use futex-based synchronisation primitives.

On versions prior to 14.4, the code should in principle default to its usual behaviour (pure spinlocks), so this should be a pretty self-contained change. Tests run with b2 cflags=-mmacosx-version-min=14.4 cxxflags=-mmacosx-version-min=14.4 pass, however, I am willing to provide something else for due process, if you have something on your mind.