fix-tests
[libc] Initial support for darwin-aarch64.
[libc] Remove the redundant header FPUtil/FEnvUtils.h
[libc] Fix small bugs in aarch64/FEnvImpl.h and simplify feclearexcept_test.
[libc] Add more robust compile time architecture detection
[libc] Change FEnv to use MXCSR as source of truth
[libc] Reset rounding mode after fsetround tests