[libc] Add support for FMA in the GPU utilities

This adds the generic FMA utilities for the GPU. We implement these
through the builtins which map to the FMA instructions in the ISA. These
may not have strict compliance with other assumptions in the the `libc`
such as rounding modes. I've included the relevant information on how
the GPU vendors map the behaviour. This should help make it easier to
implement some future generic versions.

Depends on D152486

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D152923