Support _Float128 in tgmath.h.

This patch adds tgmath.h support for _Float128, so eliminating the
awkward caveat in NEWS about the type not being supported there.  This
does inevitably increase the size of macro expansions (which grows
particularly fast when you have nested calls to tgmath.h macros), but
only when _Float128 is supported and the declarations of _Float128
interfaces are visible; otherwise the expansions are unchanged.

Tested for x86_64 and arm.

	* math/tgmath.h: Include <bits/libc-header-start.h> and
	<bits/floatn.h>.
	(__TGMATH_F128): New macro.
	(__TGMATH_CF128): Likewise.
	(__TGMATH_UNARY_REAL_ONLY): Use __TGMATH_F128.
	(__TGMATH_UNARY_REAL_RET_ONLY): Likewise.
	(__TGMATH_BINARY_FIRST_REAL_ONLY): Likewise.
	(__TGMATH_BINARY_FIRST_REAL_STD_ONLY): New macro.
	(__TGMATH_BINARY_REAL_ONLY): Use __TGMATH_F128.
	(__TGMATH_BINARY_REAL_STD_ONLY): New macro.
	(__TGMATH_BINARY_REAL_RET_ONLY): Use __TGMATH_F128.
	(__TGMATH_TERNARY_FIRST_SECOND_REAL_ONLY): Likewise.
	(__TGMATH_TERNARY_REAL_ONLY): Likewise.
	(__TGMATH_TERNARY_FIRST_REAL_RET_ONLY): Likewise.
	(__TGMATH_UNARY_REAL_IMAG): Use __TGMATH_CF128.
	(__TGMATH_UNARY_IMAG): Use __TGMATH_F128.
	(__TGMATH_UNARY_REAL_IMAG_RET_REAL): Use __TGMATH_CF128.
	(__TGMATH_BINARY_REAL_IMAG): Likewise.
	(nexttoward): Use __TGMATH_BINARY_FIRST_REAL_STD_ONLY.
	[__USE_MISC] (scalb): Use __TGMATH_BINARY_REAL_STD_ONLY.
	* math/gen-tgmath-tests.py (Type.init_types): Enable _FloatN and
	_FloatNx types if the corresponding HUGE_VAL macros are defined.