clang/AMDGPU: Do not look for rocm device libs if environment is llvm
Introduce usage of the llvm environment type. This will be useful as
a switch to eventually stop depending on externally provided libraries,
and only take bitcode from the resource directory.
I wasn't sure how to handle the confusing mess of -no-* flags. Try
to handle them all. I'm not sure --no-offloadlib makes sense for OpenCL
since it's not really offload, but interpret it anyway.
0b0dca5668