cmd/dist: bootstrapping is sequential

[4ad]

The "Building compilers and go_bootstrap" step is compiling packages sequentially (there is parallelism in running the assemblers over the .s files in a package, but packages are build one at a time). This is causing bootstrapping to be slow, especially on machines with large number of cores.

On my ARM64, which is an 8-core machine, the cmd/dist step takes 5 times longer than go install std cmd.

[rsc]

Simplicity wins here, I'm afraid.
From what you've described, go install std cmd is very fast.
That doesn't make cmd/dist too slow.