float128: Add test-{float128,ifloat128,float128-finite}

This adds test support for float128, and lays some groundwork for future
_FloatN types.

	* math/gen-libm-test.pl (@all_floats): Add ifloat128 and float128.
	(%all_floats_pfx): Add macro prefix for float128 (FLT128).
	* math/libm-test-exp10.inc (pow10_test): Do not test for _FloatN,
	* math/libm-test-isfinite.inc (finite_test): Likewise.
	* math/libm-test-lgamma.inc (gamma_test): Likewise.
	* math/libm-test-nexttoward.inc (nexttoward_test): Likewise.
	(nexttoward_test_data}: Likewise.
	* math/libm-test-remainder.inc (drem_test): Likewise.
	* math/libm-test-scalb.inc (scalb_test): Likewise.
	(scalb_test_data): Likewise.
	* math/libm-test-significand.inc (significand_test): Likewise.
	(significand_test_data): Likewise.
	* math/libm-test-support.c (check_complex): Replace __complex__
	FLOAT with CFLOAT to get the support for old compiler.
	* math/libm-test-support.h (check_complex): Likewise.
	* math/test-double.h (CFLOAT, TEST_FLOATN): New macros.
	* math/test-float.h (CFLOAT, TEST_FLOATN): Likewise.
	* math/test-ldouble.h (CFLOAT, TEST_FLOATN): Likewise.
	* math/test-float128.h: New file.
	* math/test-math-floatn.h: New file.