Allow execution of stage2 compiler to happen "later"

[alpmestan]

@bgamari brought up this ticket in a discussion we were having, and to be entirely honest this is quite likely something we want to implement in hadrian rather than the make build system. Reproducing the summary of the ticket to save everyone a click:

Currently cross-compiled bindists are rather incomplete since we can't run the stage2 compiler to build various utilities (e.g. hp2ps and haddock). Ideally we would be able to run a build up to the point where we need to run host code, package up the build tree, and finish the build on the host. There are a number of challenges that this brings, but it will ultimately make it far easier to build proper non-Linux/amd64 bindists on CircleCI (#14949).