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
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 nullptr to make things simpler.