float128: Add strtof128, wcstof128, and related functions.

float128: Add strtof128, wcstof128, and related functions.

The implementations are contained with sysdeps/ieee754/float128 as
they are only built when _Float128 is enabled within libc/m.

	* include/gmp.h (__mpn_construct_float128): New declaration.
	* include/stdlib.h: Include bits/floatn.h for _Float128 tests.
	(__strtof128_l): New declaration.
	(__strtof128_nan): Likewise.
	(__wcstof128_nan): Likewise.
	(__strtof128_internal): Likewise.
	(____strtof128_l_internal): Likewise.
	* include/wchar.h: Include bits/floatn.h for _Float128 tests.
	(__wcstof128_l): New declaration.
	(__wcstof128_internal): Likewise.

	* stdlib/Makefile (bug-strtod2): Link libm too.

	* stdlib/stdlib.h (strtof128): New declaration.
	(strtof128_l): Likewise.

	* stdlib/tst-strtod-nan-locale-main.c: Updated to use
	tst-strtod.h macros to ensure float128 gets tested too.

	* stdlib/tst-strtod-round-skeleton.c (CHOOSE_f128): New macro.

	* stdlib/tst-strtod.h: Include bits/floatn.h for _Float128
	tests.
	(IF_FLOAT128): New macro.
	(GEN_TEST_STRTOD): Update to optionally include _Float128 in
	the tests.
	(STRTOD_TEST_FOREACH): Likewise.

	* sysdeps/ieee754/float128/Makefile: Insert new strtof128 and
	wcstof128 functions into libc.

	* sysdeps/ieee754/float128/Versions: Add exports for the above
	new functions.

	* sysdeps/ieee754/float128/mpn2float128.c: New file.
	* sysdeps/ieee754/float128/strtod_nan_float128.h: New file.
	* sysdeps/ieee754/float128/strtof128.c: New file.
	* sysdeps/ieee754/float128/strtof128_l.c: New file.
	* sysdeps/ieee754/float128/strtof128_nan.c: New file.
	* sysdeps/ieee754/float128/wcstof128.c: New file.
	* sysdeps/ieee754/float128/wcstof128_l.c: New file.
	* sysdeps/ieee754/float128/wcstof128_nan.c: New fike.
	* wcsmbs/Makefile: (CFLAGS-wcstof128.c): Append strtox-CFLAGS.
	(CFLAGS-wcstof128_l): Likewise.

	* wcsmbs/wchar.h: Include bits/floatn.h for _Float128 tests.
	(wcstof128): New declaration.
	(wcstof128_l): Likewise.