[libomptarget] Enable AMDGPU devicertl
The amdgpu devicertl is written in freestanding openmp and compiles to a
bitcode library (per listed gfx arch) with no unresolved symbols. It requires
a recent clang, preferably the one from the same monorepo checkout.
This is D98658, with printf explicitly stubbed out, after patching clang to no
longer require an llvm with the amdgpu target enabled.