go/build: cgo is supported on linux/arm64 (external linking only)

Fixes #10107.

Change-Id: I309e3df7608b9eef9339196fdc50dedf5f9439f6
Reviewed-on: https://go-review.googlesource.com/8453Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

src/go/build/build.go

@@ -265,6 +265,7 @@ var cgoEnabled = map[string]bool{
 	"linux/386":       true,
 	"linux/amd64":     true,
 	"linux/arm":       true,
+	"linux/arm64":     true,
 	"linux/ppc64le":   true,
 	"android/386":     true,
 	"android/amd64":   true,