Declare some TS 18661-4 interfaces for C2X.

C2X (current version in git, postdating the most recent public draft
available as a PDF on the WG14 website) adds the interfaces from TS
18661-4, other than the reduction functions, as unconditionally
required and visible in <math.h> for hosted implementations; the
__STDC_WANT_IEC_60559_FUNCS_EXT__ macro is not included in C2X at all
(as the reduction functions aren't included at all, expected to end up
in a version of the TS updated to be relative to C2X).

This patch updates the glibc headers accordingly, following the same
pattern used for 18661-1 functions.  As the only 18661-4 functions
currently supported are the exp10 functions that have been in glibc
for a very long time, this means that in fact __GLIBC_USE
(IEC_60559_FUNCS_EXT) ends up being used only to determine the
definition of __GLIBC_USE_IEC_60559_FUNCS_EXT_C2X, not yet in any
headers other than bits/libc-header-start.h.

(I hope to add the other 18661-4 functions to glibc at some point; the
main complication is the current lack of MPFR support for many of
these functions.)

Tested for x86_64.

	* bits/libc-header-start.h (__GLIBC_USE_IEC_60559_FUNCS_EXT):
	Update comment.
	(__GLIBC_USE_IEC_60559_FUNCS_EXT_C2X): New macro.
	* bits/math-finite.h [__GLIBC_USE (IEC_60559_FUNCS_EXT)]: Change
	to [__GLIBC_USE (IEC_60559_FUNCS_EXT_C2X)].
	* math/bits/mathcalls.h [__GLIBC_USE (IEC_60559_FUNCS_EXT)]:
	Likewise.