[clang] [openmp] [OpenMP] Respect LLVM per-target install directories (PR #83282)

[Ye Luo via cfe-commits]

ye-luo wrote:

@jhuber6 unfortunately after 2fb764d2dae288f24335dfc168b5491a1017fc83
```
ls /soft/compilers/llvm/master-nightly/lib/x86_64-unknown-linux-gnu/libomptarget.rtl*
/soft/compilers/llvm/master-nightly/lib/x86_64-unknown-linux-gnu/libomptarget.rtl.cuda.so          /soft/compilers/llvm/master-nightly/lib/x86_64-unknown-linux-gnu/libomptarget.rtl.x86_64.so
/soft/compilers/llvm/master-nightly/lib/x86_64-unknown-linux-gnu/libomptarget.rtl.cuda.so.19.0git  /soft/compilers/llvm/master-nightly/lib/x86_64-unknown-linux-gnu/libomptarget.rtl.x86_64.so.19.0git

```
amdgpu plut is missing.

https://github.com/llvm/llvm-project/pull/83282