Remove build.{,cross_}compilers

[Ericson2314]

#4755 made those fields alias coredata's. But that complicates copying logic in an ad-hoc way. I don't think the complexity is worth the shorthand; it would be better to just remove those fields, and the copying special case, from build and use build.environment.coredata.{,cross_}compilers everywhere.

Not sure if we have any "new contributor" or "easy" tag, but this is a straightforward refactor, albeit one that will take a few test runs and greps to catch everything.