Summary:
This patch adds `CMAKE_REQUIRED_FLAGS` for the GPU build so checks like
`check_cxx_compiler_flags` work as expected. This is required because we
need to hack around the potential lack of `nvlink` and `ptxas` for NVPTX
targets and the fact that the AMDGPU target needs `-nogpulib` to avoid
errors on lack of ROCm. This makes a few of the checks pass and also
allows us to just check `-mcpu=native` for architecture detection
instead of finding the tools manually.
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