llvm: deparallelize build on Linux.
llvm: deparallelize build on Linux.

There appears to be a parallelisation issue in the Linux build, so let's
avoid running it in parallel. To avoid an extended serial build, let's
also skip PGO for now.

nandub/homebrew-core,lemzwerg/homebrew-core,stgraber/homebrew-core,stgraber/homebrew-core,filcab/homebrew-core,chrisfinazzo/homebrew-core,nandub/homebrew-core,Homebrew/homebrew-core,sjackman/homebrew-core,makigumo/homebrew-core,chrisfinazzo/homebrew-core,filcab/homebrew-core,makigumo/homebrew-core,adam-moss/homebrew-core,lemzwerg/homebrew-core,Homebrew/homebrew-core,adam-moss/homebrew-core,sjackman/homebrew-core