math: Enable some math builtins for clang

math: Enable some math builtins for clang

This patch enable the builtin usage for clang for the C99 functions
fpclassify, isfinite, isnormal, isnan, isinf, and sigbit.  This allows
clang optimize the calls on frontend instead of call the appropriate
glibc symbols.

Checked on aarch64-linux-gnu and x86_64-linux-gnu. I checked the supported
version for each builtin based on released version from clang/llvm.

	* math/math.h (fpclassify, isfinite, isnormal, isnan): Use builtin for
	clang 2.8.
	(signbit): Use builtin for clang 3.3.
	(isinf): Use builtin for clang 3.7.