libstdc++: Fix __floating_to_chars_precision for __float128

libstdc++: Fix __floating_to_chars_precision for __float128

The code path in __floating_to_chars_precision for handling long double
by going through printf now also handles __float128, so the condition
that guards this code path needs to get updated accordingly.

libstdc++-v3/ChangeLog:

	* src/c++17/floating_to_chars.cc (__floating_to_chars_precision):
	Relax the condition that guards the printf code path to accept
	F128_type as well as long double.