We don't need cmd/cgo for building go_bootstrap or toolchain2, so skip
building it as part of toolchain1.
This allows cmd/cgo to assume a current go/ast; e.g., that
ast.IndexListExpr exists (needed for next CL).
Change-Id: I642bba780bf273e6ea9c6e7c5d5d7ccfe86bf462
Reviewed-on: https://go-review.googlesource.com/c/go/+/353884
Trust: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>