cmd/internal/obj needs information about the default
values of GOROOT, GOARM, GOEXPERIMENT, Version, and so on.
It cannot ask package runtime, because during bootstrap
package runtime comes from Go 1.4.
So it must have its own copy.
Change-Id: I73d3e75a3d47210b3184a51a810ebb44826b81e5
Reviewed-on: https://go-review.googlesource.com/3140 Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Rob Pike <r@golang.org>