Rollup merge of #143928 - ognevny:opt-dist-build-llvm, r=Kobzol

opt-dist: make llvm builds optional

adds command line option for disabling llvm builds. it's useful in case of user having their own optimized LLVM, so they won't waste time for (at least) 3 LLVM builds. in this case PGO optimized rustc will be already built in Stage 1, so rust-lang/rust#143898 should be addressed for this change

couldn't test locally on Linux laptop due to small SSD storage, will try now with windows-msvc host

r? ``@Kobzol``

try-job: dist-x86_64-linux
try-job: dist-x86_64-msvc

Author: matthiaskrgr