Handle more _FloatN, _FloatNx types in type-generic strtod tests.

Handle more _FloatN, _FloatNx types in type-generic strtod tests.

Continuing the preparation for additional _FloatN / _FloatNx type
support, this patch adds support to type-generic strtod tests.
GEN_STRTOD_TEST_FOREACH and STRTOD_TEST_FOREACH are made to handle the
full set of such types.  tst-strtod-round-skeleton.c is updated for
those types it can handle without needing changes to the generator
(i.e. those types that have already-handled formats).

Tested for x86_64.

	* stdlib/tst-strtod.h (F16): New macro.
	(F32): Likewise.
	(F64): Likewise.
	(F32X): Likewise.
	(F64X): Likewise.
	(F128X): Likewise.
	(IF_FLOAT16): Likewise.
	(IF_FLOAT32): Likewise.
	(IF_FLOAT64): Likewise.
	(IF_FLOAT32X): Likewise.
	(IF_FLOAT64X): Likewise.
	(IF_FLOAT128X): Likewise.
	(GEN_TEST_STRTOD_FOREACH): Conditionally call macros for _Float16,
	_Float32, _Float64, _Float32x, _Float64x and _Float128x.
	(STRTOD_TEST_FOREACH): Likewise.
	* stdlib/tst-strtod-round-skeleton.c (CHOOSE_f32): New macro.
	(CHOOSE_f64): Likewise.
	(CHOOSE_f32x): Likewise.
	(CHOOSE_f64x): Likewise.