[libc] Use intrinsics for x86-64 fma and optimize PolyEval for x86-64 with...

[libc] Use intrinsics for x86-64 fma and optimize PolyEval for x86-64 with degree 3 & 5 polynomials.

- Use intrinsics for x86-64 fma
- Optimize PolyEval for x86-64 with degree 3 & 5 polynomials.
- There might be a slight loss of accuracy compared to Horner's scheme due to usages of higher powers x^2 and x^3 in the computations.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D115347