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

[Ye Luo via cfe-commits]

ye-luo wrote:

Still a myth on my side, I saw
```
[ 89%] Linking CXX shared library ../../../../../lib/libomptarget.rtl.cuda.so
[ 90%] Linking CXX shared library ../../../../../lib/libomptarget.rtl.amdgpu.so
-- Installing: /soft/compilers/llvm/main-20240305/lib/x86_64-unknown-linux-gnu/libomptarget.rtl.cuda.so.19.0git
-- Set runtime path of "/soft/compilers/llvm/main-20240305/lib/x86_64-unknown-linux-gnu/libomptarget.rtl.cuda.so.19.0git" to "$ORIGIN"
-- Installing: /soft/compilers/llvm/main-20240305/lib/x86_64-unknown-linux-gnu/libomptarget.rtl.cuda.so
```
but there is no installing of the `rtl.amdgpu.so`. I'm building llvm on a different machine.

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