[OpenMP][NVPTX] Fixed a compilation error in deviceRTLs caused by unsupported...

[OpenMP][NVPTX] Fixed a compilation error in deviceRTLs caused by unsupported feature in release verion of LLVM

`ptx71` is not supported in release version of LLVM yet. As a result,
the support of CUDA 11.2 and CUDA 11.1 caused a compilation error as mentioned
in D97004. Since the support in D97004 is just a WA for releease, and we'll not
use it in the near future, using `ptx70` for CUDA 11 is feasible.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D97195