Improve math_errhandling

Currently math_errhandling is always set to MATH_ERRNO | MATH_ERREXCEPT
even if -fno-math-errno is used.  It is not defined at all when fast-math
is used.  Set it to 0 with fast-math - this is noncomforming but more
useful than not define math_errhandling at all. Also take __NO_MATH_ERRNO__
into account and update comment.

	* math/math.h (math_errhandling): Set to 0 with __FAST_MATH__.
	Add __NO_MATH_ERRNO__ check.