Support running libm tests for float128 as alias for long double.

This patch adds support for running libm tests for float128 in the
case where the float128 functions are aliases of long double
functions.  In this case, the sysdeps Makeconfig file
(i.e. sysdeps/ieee754/ldbl-128/Makeconfig) will need to define
"float128-alias-fcts = yes" to enable the tests.

Tested for x86_64.  Also tested with build-many-glibcs.py; installed
stripped shared libraries are unchanged by the patch.  Also tested
together with changes to enable the float128 aliases.

	* math/Makefile (test-types): Add
	$(type-float128-$(float128-alias-fcts)).
	* math/test-float128.h (TYPE_STR): Define conditional on
	[FLT128_MANT_DIG == LDBL_MANT_DIG].
	(ULP_IDX): Likewise.
	(ULP_I_IDX): Likewise.