[UR][Offload] Add option to build offload adapter (#19864)
Add `--offload` option to `configure.py` which allows to build UR
offload adapter.
Because offload API is under active development in llvm-project, we have
to fetch specific version of LLVM and build offload runtime (and
dependent openmp) using those sources.
Necessary to mention that offload/openmp runtimes are built in
standalone mode using main compiler build, i.e. we don't build llvm
twice.
