[CMake] Do not append -lm to CMAKE_REQUIRED_LIBRARIES on Apple

On Apple platforms, linking against libSystem.dylib is sufficient, and
some Apple platforms don't provide libm.dylib. On those platforms, adding
-lm to CMAKE_REQUIRED_LIBRARIES causes all subsequent compile-flag checks
to fail due to the missing library.

Differential Revision: https://reviews.llvm.org/D81265