Adjust __MATH_EVAL_FMT2 definition to handle _Float16 better.

Continuing the preparation for additional _FloatN / _FloatNx type
support, this patch adds an additional case in the definition of
__MATH_EVAL_FMT2, as used in defining iseqsig: when
__FLT_EVAL_METHOD__ is 0 or 32, it adds 0.0f to the arguments, so that
the correct function would be selected in the case of _Float16
arguments with excess precision (were glibc to support _Float16, which
of course __MATH_TG and other facilities do not at present - and
_Float16 support is not part of what this patch series is aiming for,
but this particular fix is simple so is included anyway).

Tested for x86_64.

	* math/math.h
	[__FLT_EVAL_METHOD__ == 0 || __FLT_EVAL_METHOD__ == 32]
	(__MATH_EVAL_FMT2): Define to add 0.0f.