Refactor x86_64 libm code forcing underflow exceptions.

This patch refactors code in sysdeps/x86_64/fpu that forces underflow
exceptions and closely follows corresponding i386 code to use common
macros in x86_64-math-asm.h for that purpose.  This is mainly about
keeping the code similar to the i386 code as far as possible, since
each macro apart from DEFINE_LDBL_MIN ends up used only once.  It
would be possible to do a further refactoring to share these macros
between i386 and x86_64 (with i386 using the fcomip / fucomip versions
when building for i686 and above), but I have no immediate plans to do
so.

Tested for x86_64.

	* sysdeps/x86_64/fpu/x86_64-math-asm.h: New file.
	* sysdeps/x86_64/fpu/e_exp2l.S: Include <x86_64-math-asm.h>.
	(ldbl_min): Replace with use of DEFINE_LDBL_MIN.
	(__ieee754_exp2l): Use LDBL_CHECK_FORCE_UFLOW_NONNEG_NAN.
	* sysdeps/x86_64/fpu/e_expl.S: Include <x86_64-math-asm.h>.
	[!USE_AS_EXPM1L] (cmin): Replace with use of DEFINE_LDBL_MIN.
	(IEEE754_EXPL): Use LDBL_CHECK_FORCE_UFLOW_NONNEG.