[llvm] [cmake] Fix overlapping native llvm-min-tblgen builds (PR #121021)

[via llvm-commits]

arthurqiu wrote:

Inspired by https://discourse.cmake.org/t/good-way-of-running-custom-commands-serially/8412, I implemented serial execution of sub-make invocations via cmake `file(lock)`. One alternative is to use `flock`, but `flock` is not available on all platforms.

https://github.com/llvm/llvm-project/pull/121021