cmd/go: don't always link in cgo for PIE

Internal linking for PIE is now supported and enabled by default
on some platforms, for which cgo is not needed. Don't always
bring in cgo.

Change-Id: I043ed436f0e6a3acbcc53ec543f06e193d614b36
Reviewed-on: https://go-review.googlesource.com/c/go/+/261498
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: cherrymui

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

@@ -33,6 +33,7 @@ import (
 	"cmd/go/internal/search"
 	"cmd/go/internal/str"
 	"cmd/go/internal/trace"
+	"cmd/internal/sys"
 )
 
 var IgnoreImports bool // control whether we ignore imports in packages
@@ -1968,7 +1969,7 @@ func externalLinkingForced(p *Package) bool {
 	// external linking mode, as of course does
 	// -ldflags=-linkmode=external. External linking mode forces
 	// an import of runtime/cgo.
-	pieCgo := cfg.BuildBuildmode == "pie"
+	pieCgo := cfg.BuildBuildmode == "pie" && !sys.InternalLinkPIESupported(cfg.BuildContext.GOOS, cfg.BuildContext.GOARCH)
 	linkmodeExternal := false
 	if p != nil {
 		ldflags := BuildLdflags.For(p)

src/cmd/internal/sys/supported.go

@@ -115,3 +115,13 @@ func BuildModeSupported(compiler, buildmode, goos, goarch string) bool {
 		return false
 	}
 }
+
+func InternalLinkPIESupported(goos, goarch string) bool {
+	switch goos + "/" + goarch {
+	case "linux/amd64", "linux/arm64",
+		"android/arm64",
+		"windows-amd64", "windows-386", "windows-arm":
+		return true
+	}
+	return false
+}