HIP: Add -munsafe-fp-atomics to CMake

[ax3l]

Summary

Otherwise atomicAdd is much slower. Same as already done in GNU Make #2567

Additional background

• HIP: Add -munsafe-fp-atomics to GNU Make #2567
• https://reviews.llvm.org/D91546?id=305522
• https://reviews.llvm.org/D97967 (https://llvm-gpu-news.github.io/2021/03/05/issue-7.html)

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[ax3l]

Hm, I see:

clang-12: error: unknown argument: '-munsafe-fp-atomics'

in HiPACE++ now: Hi-PACE/hipace#644

[ax3l]

Ah, we still used HIP 4.1 in CI: Hi-PACE/hipace#645