We cannot use the PWD trick for any compile on Darwin, since /proc
doesn't exist. So instead of checking for darwin host modules, just
check runtime.GOOS.
And since this isn't a per-module decision, don't pass it along as a
variable to every build command, but make it a global variable.
Change-Id: Iea8609f49a9d316c58aed527f62d1986c970eaac
322a0a6b59