[clang] [llvm] [LLVM] Add GNU make jobserver support (PR #145131)

[Artem Belevich via cfe-commits]

https://github.com/Artem-B approved this pull request.

LGTM syntax/style-wise. Looks reasonable on the functionality side, but we could use a second opinion on that.

https://github.com/llvm/llvm-project/pull/145131