Remove the FMA preventer and avoid FMA for Clang in core-math

[pleroy]

#1760.

[eggrobin]

1. I think this makes the functions incorrect in MSVC debug and on clang; we should call std::fma if we really cannot use FMA;
2. Clang has a __builtin_fma, which calls std::fma if there is no FMA available: https://godbolt.org/z/814Ebdanr.

Suggested change

	#define __builtin_fma(x, y, z) \
	(principia::numerics::_fma::UseHardwareFMA \
	? principia::numerics::_fma::FusedMultiplyAdd(x, y, z) \
	: x * y + z)
	#if PRINCIPIA_COMPILER_MSVC
	#define __builtin_fma(x, y, z) principia::numerics::_fma::FusedMultiplyAdd(x, y, z)
	#endif