Use __HAVE_DISTINCT_FLOAT128 in tgmath.h.

When _Float128 is ABI-equivalent to long double, there is no need for
tgmath.h to have any special _Float128 handling: it's always OK to
call the long double versions of functions for _Float128 arguments in
that case, and the logic to determine return types is generic.  Thus,
this patch changes the use of __HAVE_FLOAT128 to
__HAVE_DISTINCT_FLOAT128, as a minor optimization to reduce the size
of the macro expansions in the ABI-equivalent case.

Tested for x86_64.

	* math/tgmath.h [__HAVE_FLOAT128]: Change conditional to
	[__HAVE_DISTINCT_FLOAT128].