cmd/go,cmd/internal/sys,cmd/link: skip Go build ids for externally linked tools

cmd/go already skips build ids on Android where buildmode=pie is
forced. Expand the check to all externally linked tools.

Necessary for self-hosted iOS builds where PIE is not forced but
external linking is.

Updates #31722

Change-Id: Iad796a9411a37eb0c44d365b70a3c5907537e461
Reviewed-on: https://go-review.googlesource.com/c/go/+/174307
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/work/gc.go

@@ -542,11 +542,11 @@ func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string)
 	// Store BuildID inside toolchain binaries as a unique identifier of the
 	// tool being run, for use by content-based staleness determination.
 	if root.Package.Goroot && strings.HasPrefix(root.Package.ImportPath, "cmd/") {
-		// When buildmode=pie, external linking will include our build
-		// id in the external linker's build id, which will cause our
-		// build id to not match the next time the tool is built.
+		// External linking will include our build id in the external
+		// linker's build id, which will cause our build id to not
+		// match the next time the tool is built.
 		// Rely on the external build id instead.
-		if ldBuildmode != "pie" || !sys.PIEDefaultsToExternalLink(cfg.Goos, cfg.Goarch) {
+		if !sys.MustLinkExternal(cfg.Goos, cfg.Goarch) {
 			ldflags = append(ldflags, "-X=cmd/internal/objabi.buildID="+root.buildID)
 		}
 	}

src/cmd/internal/sys/supported.go

@@ -31,10 +31,15 @@ func MSanSupported(goos, goarch string) bool {
 	}
 }
 
-// PIEDefaultsToExternalLink reports whether goos/goarch defaults
-// to external linking for buildmode=pie.
-func PIEDefaultsToExternalLink(goos, goarch string) bool {
-	// Currently all systems external link PIE binaries.
-	// See https://golang.org/issue/18968.
-	return true
+// MustLinkExternal reports whether goos/goarch requires external linking.
+func MustLinkExternal(goos, goarch string) bool {
+	switch goos {
+	case "android":
+		return true
+	case "darwin":
+		if goarch == "arm" || goarch == "arm64" {
+			return true
+		}
+	}
+	return false
 }

src/cmd/link/internal/ld/config.go

@@ -175,13 +175,8 @@ func mustLinkExternal(ctxt *Link) (res bool, reason string) {
 		}()
 	}
 
-	switch objabi.GOOS {
-	case "android":
-		return true, "android"
-	case "darwin":
-		if ctxt.Arch.InFamily(sys.ARM, sys.ARM64) {
-			return true, "iOS"
-		}
+	if sys.MustLinkExternal(objabi.GOOS, objabi.GOARCH) {
+		return true, fmt.Sprintf("%s/%s requires external linking", objabi.GOOS, objabi.GOARCH)
 	}
 
 	if *flagMsan {
@@ -256,7 +251,7 @@ func determineLinkMode(ctxt *Link) {
 				ctxt.LinkMode = LinkExternal
 			} else if iscgo && externalobj {
 				ctxt.LinkMode = LinkExternal
-			} else if ctxt.BuildMode == BuildModePIE && sys.PIEDefaultsToExternalLink(objabi.GOOS, objabi.GOARCH) {
+			} else if ctxt.BuildMode == BuildModePIE {
 				ctxt.LinkMode = LinkExternal
 			} else {
 				ctxt.LinkMode = LinkInternal