libstdc++: Support std::abs for 128-bit integers and floats [PR96710]

libstdc++: Support std::abs for 128-bit integers and floats [PR96710]

Currently we only provide std::abs(__int128) and std::abs(__float128)
for non-strict modes, i.e. -std=gnu++NN but not -std=c++NN.

This defines those overloads for strict modes too, as a small step
towards resolving PR 96710 (which will eventually mean that __int128
satisfies the std::integral concept).

libstdc++-v3/ChangeLog:

	PR libstdc++/96710
	* include/bits/std_abs.h [__SIZEOF_INT128__] (abs(__int128)):
	Define.
	[_GLIBCXX_USE_FLOAT128] (abs(__float128)): Enable definition for
	strict modes.
	* testsuite/26_numerics/headers/cmath/82644.cc: Use strict_std
	instead of defining __STRICT_ANSI__.
	* testsuite/26_numerics/headers/cstdlib/abs128.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>