Problem statement:

Currently cmd/dist uses hard coded defaults for the number of concurrent jobs to execute
during the early stages of ./make.bash; up to the point of go_bootstrap taking over.

This number is 1 on arm machines and 4 everywhere else. For arm specifically this value
used to be correct 3 years ago but now dual and quad core arm host are available this
unnecessarily delays an already slow c compilation phase.

This value can be easily changed after checkout, but as the builders always build from a
clean tree this is not an option.

It may also be for large x86 machines the value of 4 parallel jobs is too conservative,
or to high if operating in a virtualised environment, however this is not the main focus
of this proposal.

Implementation:

I propose to add a new flag cmd/dist which would override the default value. If not set
the current defaults of 4 (or 1 for arm) would apply.

The proposed name of this flag is -j.

This flag would also need to be passed through from the builder command via the existing
GO_DISTFLAGS env var.

Builder owners can set GO_DISTFLAGS to control the number of parallel jobs during the
build allowing them to improve build times.

Once the build arrives at go_bootstrap the current behaviour of running runtime.NumCPU()
jobs would take over. This does not change as part of this proposal.

See also: https://groups.google.com/d/msg/golang-dev/GjhQspYudHk/vNVFsd_49AEJ