cmd/go: -ldflags=-linkmode=external requires runtime/cgo

We add runtime/cgo to the list of import paths for various cases that
imply external linking mode, but before this change we did not add for
an explicit request of external linking mode. This fixes the case where
you are using a non-default buildmode that implies a different
compilation option (for example, -buildmode=pie implies -shared) and the
runtime/cgo package for that option is stale.

No test, as I'm not sure how to write one. It would require forcing a
stale runtime/cgo.

Change-Id: Id0409c7274ce67fe15d910baf587d3220cb53d83
Reviewed-on: https://go-review.googlesource.com/33070Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

src/cmd/go/pkg.go

@@ -913,11 +913,22 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		importPaths = append(importPaths, "syscall")
 	}
 
-	// Currently build modes c-shared, pie, plugin, and -linkshared force
-	// external linking mode, and external linking mode forces an
-	// import of runtime/cgo.
+	// Currently build modes c-shared, pie (on systems that do not
+	// support PIE with internal linking mode), plugin, and
+	// -linkshared force external linking mode, as of course does
+	// -ldflags=-linkmode=external. External linking mode forces
+	// an import of runtime/cgo.
 	pieCgo := buildBuildmode == "pie" && (buildContext.GOOS != "linux" || buildContext.GOARCH != "amd64")
-	if p.Name == "main" && !p.Goroot && (buildBuildmode == "c-shared" || buildBuildmode == "plugin" || pieCgo || buildLinkshared) {
+	linkmodeExternal := false
+	for i, a := range buildLdflags {
+		if a == "-linkmode=external" {
+			linkmodeExternal = true
+		}
+		if a == "-linkmode" && i+1 < len(buildLdflags) && buildLdflags[i+1] == "external" {
+			linkmodeExternal = true
+		}
+	}
+	if p.Name == "main" && !p.Goroot && (buildBuildmode == "c-shared" || buildBuildmode == "plugin" || pieCgo || buildLinkshared || linkmodeExternal) {
 		importPaths = append(importPaths, "runtime/cgo")
 	}