cmd/go: -buildmode=pie forces external linking mode on all systems

The go tool assumed that -buildmode=pie implied internal linking on
linux-amd64. However, that was changed by CL 36417 for issue #18968.

Fixes #21452

Change-Id: I8ed13aea52959cc5c53223f4c41ba35329445545
Reviewed-on: https://go-review.googlesource.com/57231
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -953,11 +953,12 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) *Package
 
 	if cfg.BuildContext.CgoEnabled && p.Name == "main" && !p.Goroot {
 		// 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
+		// support PIE with internal linking mode (currently all
+		// systems: issue #18968)), plugin, and -linkshared force
+		// external linking mode, as of course does
 		// -ldflags=-linkmode=external. External linking mode forces
 		// an import of runtime/cgo.
-		pieCgo := cfg.BuildBuildmode == "pie" && (cfg.BuildContext.GOOS != "linux" || cfg.BuildContext.GOARCH != "amd64")
+		pieCgo := cfg.BuildBuildmode == "pie"
 		linkmodeExternal := false
 		for i, a := range cfg.BuildLdflags {
 			if a == "-linkmode=external" {