float128: Add private _Float128 declarations for libm.

float128: Add private _Float128 declarations for libm.

Add the necessary bits to the private headers to support
building the _Float128 libm functions.

A local override for float.h is provided to include the
missing *FLT128 macros implied by TS 18661-3 for this
type when compiling prior to GCC 7.

	* include/complex.h (__kernel_casinhf128): New declaration.
	* include/float.h: New file.
	* include/math.h (__finitef128): Add a hidden def.
	(__isinff128): Likewise.
	(__isnanf128): Likewise.
	(__fpclassify): Likewise.
	(__issignalling): Likewise.
	(__expf128): Likewise.
	(__expm1f128): Likewise.

	* sysdeps/generic/fix-fp-int-convert-overflow.h:
	(FIX_FLT128_LONG_CONVERT_OVERFLOW): New macro.
	(FIX_FLT128_LLONG_CONVERT_OVERFLOW): Likewise.

	* sysdeps/generic/math-type-macros-float128.h: New file.

	* sysdeps/generic/math_private.h: Include bits/floatn.h and
	math_private_calls.h for _Float128.
	(__isinff128): New inline implementation used when GCC < 7.0,
	since in this case __builtin_isinf_sign is broken.
	(fabsf128): New inline implementation that calls the builtin.
	(__EXPR_FLT128): New macro.
	(min_of_type): Optionally include _Float128 types too.

	* sysdeps/generic/math_private_calls.h (__kernel_sincos):
	Declare for _Float128.
	(__kernel_rem_pio2): Likewise.

	* sysdeps/ieee754/ldbl-opt/s_sin.c:
	(__DECL_SIMD_sincos_disablef128): New macro.