[offload] Permit redefining OPENMP_STANDALONE_BUILD (#157253)
Permit redefining `OPENMP_STANDALONE_BUILD` to make it possible to build
offload correctly via runtimes build (i.e. build where the top-level
project is `runtimes`). This follows the same logic in `openmp`
component.
Signed-off-by: Michał Górny <mgorny@gentoo.org>