Also handle __STDC_WANT_IEC_60559_BFP_EXT__ in <tgmath.h>.

My __STDC_WANT_IEC_60559_BFP_EXT__ patch omitted to update the
conditions on the nextup and nextdown type-generic macros in
<tgmath.h>.  This patch updates those conditions accordingly.  (As
glibc doesn't currently have an exp10 type-generic macro, no such
changes are needed relating to __STDC_WANT_IEC_60559_FUNCS_EXT__;
adding such a type-generic macro would be a new feature.)

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

	* math/tgmath.h (nextdown): Define if
	[__GLIBC_USE (IEC_60559_BFP_EXT)], not if [__USE_GNU].
	(nextup): Likewise.