Error out when building XLA's CPU and GPU backends with fast-math

In an ideal world this won't make a difference since the compiler should be
disciplined about not leaking host-level optimization artifacts into generated
code.  However, I think this provides some defense-in-depth in preventing
fast-math optimization on the host side from messing up floating point constants
etc. we want to embed into generated code.

PiperOrigin-RevId: 185941549