libstdc++: Fix std::abs(__float128) for -NaN and -0.0 [PR109758]

The current implementation of this non-standard overload of std::abs
incorrectly returns a negative value for negative NaNs and negative
zero, because x < 0 is false in both cases.

Use fabsl(long double) or fabsf128(_Float128) if those do the right
thing.  Otherwise, use __builtin_signbit(x) instead of x < 0 to detect
negative inputs. This assumes that __builtin_signbit handles __float128
correctly, but that seems to be true for all of GCC, clang and icc.

libstdc++-v3/ChangeLog:

	PR libstdc++/109758
	* include/bits/std_abs.h (abs(__float128)): Handle negative NaN
	and negative zero correctly.
	* testsuite/26_numerics/headers/cmath/109758.cc: New test.

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