libstdc++: Format __float128 as _Float128 only when long double is not 128 IEEE ...

libstdc++: Format __float128 as _Float128 only when long double is not 128 IEEE [PR120976]

For powerpc64 and sparc architectures that both have __float128 and 128bit long double,
the __float128 is same type as long double/__ieee128 and already formattable.

The remaining specialization makes __float128 formattable on x86_64 via _Float128,
however __float128 is now not formattable on x86_32 (-m32) with -mlong-double-128,
where __float128 is distinct type from long double that is 128bit IEEE.

	PR libstdc++/120976

libstdc++-v3/ChangeLog:

	* include/std/format (formatter<__float128, _Char_T): Define if
	_GLIBCXX_FORMAT_F128 == 2.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>