[AMDGPU] Add option -munsafe-fp-atomics

Add an option -munsafe-fp-atomics for AMDGPU target.

When enabled, clang adds function attribute "amdgpu-unsafe-fp-atomics"
to any functions for amdgpu target. This allows amdgpu backend to use
unsafe fp atomic instructions in these functions.

Differential Revision: https://reviews.llvm.org/D91546