[build] Allow automatically applying Julia's patches to LLVM source

[giordano]

This is useful when one wants to build Julia with a non-Julia fork of LLVM, but still apply Julia's patches, to be a good citizen. I'm happy to document these new options in the devdocs if #50207 is accepted.

[DilumAluthge]

Would it be possible to eventually add the same feature for other dependencies that we have custom forks of, such as libuv?

[giordano]

We don't have stable tags for Julia's libuv, which is a major pain point for downstream packagers like spack, so the answer is no. But also, I'm not sure libuv is so commonly forked as llvm is, for which one may want to compile a fork with a new backend enabled not yet available in upstream (which was the main motivation for my PR).