As suggested by Jakub in the PR, this just hardcodes the constants with
a Q suffix, since the properties of __float128 are not going to change.

We can only define it for non-strict modes because the suffix gives an
error otherwise, even in system headers:

limits:2085: error: unable to find numeric literal operator 'operator""Q'

libstdc++-v3/ChangeLog:

	PR libstdc++/104772
	* include/std/limits (numeric_limits<__float128>): Define.
	* testsuite/18_support/numeric_limits/128bit.cc: New test.