Add amrex::Math::rsqrt

[AlexanderSinn]

Summary

This PR adds a function to compute the inverse square root of a single- or double-precision value. On GPUs this should be faster than doing the inverse and square root separately.

Additional background

https://godbolt.org/z/sb8P3TMvq

https://docs.nvidia.com/cuda/cuda-c-best-practices-guide/index.html#reciprocal-square-root

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