s390x SIMD: update abs() function for complex numbers (#108515)
It propagated NANs when it should have not.
Also, replace it with std::abs due to precision mismatching.
This change fixes test_python_ref__refs_abs_cpu_complex32 test in test/test_ops.py.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108515
Approved by: https://github.com/ezyang
