Limit the number of parallel link jobs during LLVM build for mingw. This PR is an attempt to unblock https://github.com/rust-lang/rust/pull/108355, which keeps failing while trying to link various LLVM artifacts on mingw runners. It looks like doing too many linking jobs might put too much load on the system? (Although I don't understand why the jobs are only failing for #108355 while they seem to pass for others) r? infra-ci