Declare most TS 18661-1 interfaces for C2X.

Declare most TS 18661-1 interfaces for C2X.

C2X adds the interfaces from TS 18661-1, and all except a handful in
Annex F are unconditionally visible in C2X rather than only visible
when __STDC_WANT_IEC_60559_BFP_EXT__ is defined.  This patch updates
glibc headers accordingly: most uses of __GLIBC_USE
(IEC_60559_BFP_EXT) are changed to a new __GLIBC_USE
(IEC_60559_BFP_EXT_C2X).  (Regarding totalorder and totalordermag, the
type-generic macros in tgmath.h will go away when the functions are
changed to take pointer arguments.)

	* bits/libc-header-start.h (__GLIBC_USE_IEC_60559_BFP_EXT): Update
	comment.
	(__GLIBC_USE_IEC_60559_BFP_EXT_C2X): New macro.
	* bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Change to
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)].
	* include/limits.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Likewise.
	* math/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Likewise.
	* math/math.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Likewise.
	* stdlib/bits/stdlib-ldbl.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* stdlib/stdint.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Likewise.
	* stdlib/stdlib.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Likewise.
	* sysdeps/aarch64/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/alpha/fpu/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/arm/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/csky/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/hppa/fpu/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/ia64/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/m68k/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/microblaze/bits/fenv.h [__GLIBC_USE
	(IEC_60559_BFP_EXT)]: Likewise.
	* sysdeps/mips/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/nios2/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/powerpc/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/riscv/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/s390/fpu/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/sh/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/sparc/fpu/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* sysdeps/x86/fpu/bits/fenv.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise.
	* math/bits/mathcalls.h [__GLIBC_USE (IEC_60559_BFP_EXT)]:
	Likewise, except for totalorder, totalordermag, getpayload,
	setpayload and setpayloadsig.
	* math/tgmath.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Likewise,
	except for totalorder and totalordermag.