Build of Go's runtime package fails when cross-compiling with go1.9.4 #733

Cross-compiling a program that depends on Go's runtime package using the following compilation flags on a amd64/Linux system:

CGO_ENABLED=0 GOOS=freebsd GOARCH=amd64

It works when just using gb build for my native platform.

To reproduce:
Compile this sample program from the Go runtime example with the following directory structure:

src
|__ app
    |__ main.go

GOROOT=/path/to/go1.9.2 CGO_ENABLED=0 GOOS=freebsd GOARCH=amd64 gb build works.
GOROOT=/path/to/go1.9.4 CGO_ENABLED=0 GOOS=freebsd GOARCH=amd64 gb build fails with the error:

# runtime
cgo.go:9:3: //go:cgo_export_static main only allowed in cgo-generated code

According to Ian Lance Taylor, this error was newly introduced in Go 1.9.4. See: https://groups.google.com/forum/#!topic/golang-nuts/52ZHzgo6u1k

Edit: This is with a freshly-built version of gb, built with go 1.9.4.