[OpenMP] Make `-fopenmp-target=` use the `nvptx-arch` tool

Previously, if the user did not provide an architecture when using
`-fopenmp-targets=nvptx64` we used the value from
`CLANG_OPENMP_DEFAULT_NVPTX_ARCH` which is defined at compile time. This
isn't ideal because it means that the default is set when the LLVM
compiler it built. Instead this patch uses the `nvptx-arch` tool to
query it at runtime. This matches the existing behaviour of the AMDGPU
toolchain with its `amdgpu-arch` tool.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D141708