Support __STDC_WANT_IEC_60559_FUNCS_EXT__ feature test macro.

Support __STDC_WANT_IEC_60559_FUNCS_EXT__ feature test macro.

This patch implements support for the
__STDC_WANT_IEC_60559_FUNCS_EXT__ feature test macro, following the
__GLIBC_USE approach used for other ISO C feature test macros.
Currently this only affects the exp10 functions (which glibc has had
for a long time).

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).

	* bits/libc-header-start.h (__GLIBC_USE_IEC_60559_FUNCS_EXT): New
	macro.
	* include/features.h (__STDC_WANT_IEC_60559_FUNCS_EXT__):
	Document.
	* manual/creature.texi (__STDC_WANT_IEC_60559_FUNCS_EXT__):
	Document macro.
	* manual/math.texi (exp10): Document as ISO from TS 18661-4:2015.
	(exp10f): Likewise.
	(exp10l): Likewise.
	* math/bits/mathcalls.h (exp10): Declare if
	[__GLIBC_USE (IEC_60559_FUNCS_EXT)], not [__USE_GNU].