cmd/link: use gold when dynamic linking on arm64

The GNU linker follows the letter of -znocopyreloc by refusing to
generate COPY relocations on arm64. Unfortunately it generates an
error instead of finding another way. The gold linker works, so
switch to it.

Fixes linux/arm64 build.

Change-Id: I1f7119d999c8f9f1f2d0c1e06b6462cea9c02a71
Reviewed-on: https://go-review.googlesource.com/22185
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/testshared/shared_test.go

@@ -329,21 +329,21 @@ func readNotes(f *elf.File) ([]*note, error) {
 	return notes, nil
 }
 
-func dynStrings(path string, flag elf.DynTag) []string {
+func dynStrings(t *testing.T, path string, flag elf.DynTag) []string {
 	f, err := elf.Open(path)
 	defer f.Close()
 	if err != nil {
-		log.Fatal("elf.Open failed: ", err)
+		t.Fatalf("elf.Open(%q) failed: %v", path, err)
 	}
 	dynstrings, err := f.DynString(flag)
 	if err != nil {
-		log.Fatal("dynstring failed: ", err)
+		t.Fatalf("DynString(%s) failed on %s: %v", flag, path, err)
 	}
 	return dynstrings
 }
 
 func AssertIsLinkedToRegexp(t *testing.T, path string, re *regexp.Regexp) {
-	for _, dynstring := range dynStrings(path, elf.DT_NEEDED) {
+	for _, dynstring := range dynStrings(t, path, elf.DT_NEEDED) {
 		if re.MatchString(dynstring) {
 			return
 		}
@@ -357,7 +357,7 @@ func AssertIsLinkedTo(t *testing.T, path, lib string) {
 
 func AssertHasRPath(t *testing.T, path, dir string) {
 	for _, tag := range []elf.DynTag{elf.DT_RPATH, elf.DT_RUNPATH} {
-		for _, dynstring := range dynStrings(path, tag) {
+		for _, dynstring := range dynStrings(t, path, tag) {
 			for _, rpath := range strings.Split(dynstring, ":") {
 				if filepath.Clean(rpath) == filepath.Clean(dir) {
 					return

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

@@ -1123,10 +1123,15 @@ func hostlink() {
 		// from the beginning of the section (like STYPE).
 		argv = append(argv, "-Wl,-znocopyreloc")
 
-		if SysArch.Family == sys.ARM {
-			// The GNU linker will generate COPY relocations on ARM
-			// even with -znocopyreloc set. Switch to gold.
+		if SysArch.InFamily(sys.ARM, sys.ARM64) {
+			// On ARM, the GNU linker will generate COPY relocations
+			// even with -znocopyreloc set.
 			// https://sourceware.org/bugzilla/show_bug.cgi?id=19962
+			//
+			// On ARM64, the GNU linker will fail instead of
+			// generating COPY relocations.
+			//
+			// In both cases, switch to gold.
 			argv = append(argv, "-fuse-ld=gold")
 		}
 	}