Support parallel front end test in rustc-perf

[SparrowLii]

Parallel front end is an important means to improve compiler performance, but currently we cannot use rustc-perf to test the impact on the compiler performance of the parallelized front end in a PR.

We should support the -Z threads=n compiler option when compiling the use cases in the bot to compare the impact of code changes in PRs on the parallelization frontend.

updates #118698
updates #113349

cc @Kobzol @jieyouxu