libstdc++: Fix up to_chars ppc64le _Float128 overloads [PR107636]

As reported, I've misplaced __extension__ keywords in these cases
(wanted not to have them on the whole inlines because _Float128 is
completely standard now while __float128 is not, but before return
it is a syntax error.
I've verified on a short testcase that both g++ and clang++ accept
__extension__ after return keyword.

2022-11-12  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/107636
	* include/std/charconv (to_chars): Fix up powerpc64le _Float128
	overload __extension__ placement.