libstdc++: Add std::formatter specializations for extended float types

This makes it possible to format _Float32, _Float64 etc. in C++20 mode.
Previously it was only possible to format them in C++23 when the
<stdfloat> typedefs and the std::to_chars overloads were defined.

Instead of relying on std::to_chars for those types, we can just reuse
the formatters for float, double and long double. This also avoids
template bloat by reusing the same specializations instead of
instantiating __formatter_fp for every different type.

libstdc++-v3/ChangeLog:

	* include/std/format (formatter): Add partial specializations
	for extended floating-point types.
	* testsuite/std/format/functions/format.cc: Move test_float128()
	to ...
	* testsuite/std/format/formatter/ext_float.cc: New test.