Flang-new needs to add `mlink-builtin-bitcode` objects to properly
support offload code generation for AMD GPUs (for example, math
functions).
Both Flang-new and Clang rely on `mlink-builtin-bitcode` flags. These
flags are added by the `AMDGPUOpenMPToolchain::addClangTargetOptions`
function. Now, both compilers reuse the same function.
Flang-new tests for AMDGPU were updated by adding the `-nogpulib` flag.
This flag allows running AMDGPU tests on machines without the ROCm stack.
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