[llvm] [openmp] [Offload] Remove standalone build in favor of 'runtimes' (PR #162904)

[Michał Górny via llvm-commits]

mgorny wrote:

I'm sorry, I misread your comment. If I pass `-DLLVM_BINARY_DIR=/usr/lib/llvm/22/bin`, then I get the same error, then everything explodes over trying to write there:

```
CMake Error at /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload/plugins-nextgen/common/CMakeLists.txt:20 (add_dependenci
es):
  The dependency target "intrinsics_gen" of target "PluginCommon" does not
  exist.


 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
 * ACCESS DENIED:  mkdir:              /usr/lib/llvm/22/bin/bin
```

(these are our PM errors over access violations)

https://github.com/llvm/llvm-project/pull/162904