Optimize atomic floating-point operations

[eschnett]

LLVM 9 provides built-ins for atomic floating-point operations (see http://releases.llvm.org/9.0.0/docs/ReleaseNotes.html). Julia should use these in Base/atomics.jl, as they are likely faster than the current implementation using atomic_cas!.

[vchuravy]

Atomic floating-point operations are not supported by all backends so we would need an atomic-expand pass for them

[gbaraldi]

I think we currently lower all atomics to CAS loops, #45122 is an attempt at not doing that, but it had some issues. And yeah, we would need to teach it to fall back to the CAS if it's not supported (I'm not sure LLVM is able to do that by itself)