Update compiler specs on RZAnsel

[gzagaris]

On RZAnsel plaform, we are currently using:

• [email protected]
• IBM XL C/C++ 2019.08.20
• NVIDIA CUDA 10.1.243
• [email protected]

We should update to:

• clang10
• IBM XL C/C++ 2020.09.17 (there have been some bugfixes to the compiler)
• NVIDA CUDA 11

We may also want to revisit if we want a GCC spec on this platform (I think it would be a less common option) and consider either removing the GCC spec or updating it.

[rhornung67]

Survey code teams to see what compilers they are using. Stick with what they are using in day-to-day development and production uses.

[rhornung67]

Focus on this after the release.

[kennyweiss]

Closing this issue. Will create a new one to move to compilers from shared radiuss-spack configs.

[white238]

#1055