If you are, say, on a Mac, and you
cd src/pkg/runtime
GOOS=linux make
then cmd/dist correctly builds the linux copy of runtime.a.
However, if you
cd src/cmd/6l
GOOS=linux make
then cmd/dist incorrectly tries to build a "linux" copy of 6l (and fails,
beacuse the "linux" copies of the libraries don't exist). It should be
building the darwin copy, because GOHOSTOS is darwin.
The make.bash sequence sets GOHOSTOS=GOOS and GOHOSTARCH=GOARCH during the builds in
those directories, so it hasn't come up in that context. However, if you are working on
a cross-compiled system it's annoying that you can't leave GOOS set to the target
system. Every time you build a host tool you need to set it back.
