[llvm][nvptx] Add sm_90a

This works adds `sm_90a` as nvptx target. `sm_90a` is required to generate  wgmma and setmaxnreg instructions.

Here is information about "a" prefix in PTX document:
Target architectures with suffix “a”, such as sm_90a, include architecture-accelerated features that are supported on the specified architecture only, hence such targets do not follow the onion layer model. Therefore, PTX code generated for such targets cannot be run on later generation devices. Architecture-accelerated features can only be used with targets that support these features.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D155851