[CUDA][HIP] Fix bound arch for offload action for fat binary

Currently CUDA/HIP toolchain uses "unknown" as bound arch
for offload action for fat binary. This causes -mcpu or -march
with "unknown" added in HIPToolChain::TranslateArgs or
CUDAToolChain::TranslateArgs.

This causes issue for https://reviews.llvm.org/D88377 since
HIP toolchain needs to check -mcpu in HIPToolChain::TranslateArgs.

The bound arch of offload action for fat binary is not really
used, therefore set it to CudaArch::UNUSED.

Differential Revision: https://reviews.llvm.org/D88524