Adds MSVC intrinsics for atomic exchange and compare & exchange.
These were needed while I was working on porting CPython to clang-cl.
They are documented here:
https://docs.microsoft.com/en-us/cpp/intrinsics/interlockedcompareexchange-intrinsic-functions
https://docs.microsoft.com/en-us/cpp/intrinsics/interlockedexchange-intrinsic-functions
I'm not entirely sure what type of test case is needed for this, so any guidance on that would be appreciated.