[CUDA] Add getTargetFeatures for the NVPTX toolchain

The NVPTX toolchain uses target features to determine the PTX version to
use. However this isn't exposed externally like most other toolchain
specific target features are. Add this functionaliy in preparation for
using it in for OpenMP offloading.

Reviewed By: jdoerfert, tra

Differential Revision: https://reviews.llvm.org/D122089