Rollup merge of #150108 - ZuseZ4:offload-build-rework, r=Kobzol

Offload: Build offload as a single Step

r? `@Kobzol`

Since it looks like we'll postpone enabling offload in CI for a bit, I factored out these improvements which we want independently. I locally tested both build options successfully, the in-tree-clang build, as well as the build where we provide a path to an external clang.

Author: matthiaskrgr