cmd/link: default to internal linking for android/arm64

The bootstrapping process (make.bash) on all other platforms use
internal linking. This change brings android/arm64 in line, fixing the
scary warning on our self-hosted Corellium builders:

warning: unable to find runtime/cgo.a

The linkmode default is changed to internal for all Android programs,
but in practice that won't matter outside our builders: using Go with
Android apps requires buildmode=c-shared which uses linkmode external.

Fixes #31343
Updates #31819

Change-Id: I3b3ada5ed69a7989e6d8e5960bbebf5e1c22aada
Reviewed-on: https://go-review.googlesource.com/c/go/+/207299
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1871,7 +1871,9 @@ func externalLinkingForced(p *Package) bool {
 	// Some targets must use external linking even inside GOROOT.
 	switch cfg.BuildContext.GOOS {
 	case "android":
-		return true
+		if cfg.BuildContext.GOARCH != "arm64" {
+			return true
+		}
 	case "darwin":
 		switch cfg.BuildContext.GOARCH {
 		case "arm", "arm64":

src/cmd/internal/sys/supported.go

@@ -35,7 +35,9 @@ func MSanSupported(goos, goarch string) bool {
 func MustLinkExternal(goos, goarch string) bool {
 	switch goos {
 	case "android":
-		return true
+		if goarch != "arm64" {
+			return true
+		}
 	case "darwin":
 		if goarch == "arm" || goarch == "arm64" {
 			return true

src/cmd/link/internal/arm64/obj.go

@@ -57,7 +57,8 @@ func Init() (*sys.Arch, ld.Arch) {
 		Gentext:          gentext,
 		Machoreloc1:      machoreloc1,
 
-		Linuxdynld: "/lib/ld-linux-aarch64.so.1",
+		Androiddynld: "/system/bin/linker64",
+		Linuxdynld:   "/lib/ld-linux-aarch64.so.1",
 
 		Freebsddynld:   "/usr/libexec/ld-elf.so.1",
 		Openbsddynld:   "/usr/libexec/ld.so",

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

@@ -189,6 +189,9 @@ func mustLinkExternal(ctxt *Link) (res bool, reason string) {
 	if iscgo && ctxt.Arch.InFamily(sys.MIPS64, sys.MIPS, sys.PPC64) {
 		return true, objabi.GOARCH + " does not support internal cgo"
 	}
+	if iscgo && objabi.GOOS == "android" {
+		return true, objabi.GOOS + " does not support internal cgo"
+	}
 
 	// When the race flag is set, the LLVM tsan relocatable file is linked
 	// into the final binary, which means external linking is required because
@@ -205,7 +208,7 @@ func mustLinkExternal(ctxt *Link) (res bool, reason string) {
 		return true, "buildmode=c-shared"
 	case BuildModePIE:
 		switch objabi.GOOS + "/" + objabi.GOARCH {
-		case "linux/amd64", "linux/arm64":
+		case "linux/amd64", "linux/arm64", "android/arm64":
 		default:
 			// Internal linking does not support TLS_IE.
 			return true, "buildmode=pie"
@@ -244,10 +247,16 @@ func determineLinkMode(ctxt *Link) {
 			ctxt.LinkMode = LinkExternal
 			via = "via GO_EXTLINK_ENABLED "
 		default:
-			if extNeeded || (iscgo && externalobj) {
+			ctxt.LinkMode = LinkInternal
+			switch {
+			case extNeeded, iscgo && externalobj:
 				ctxt.LinkMode = LinkExternal
-			} else {
-				ctxt.LinkMode = LinkInternal
+			case ctxt.BuildMode == BuildModePIE:
+				// Android always use BuildModePIE, and needs internal linking for
+				// bootstrapping.
+				if objabi.GOOS != "android" || objabi.GOARCH != "arm64" {
+					ctxt.LinkMode = LinkExternal
+				}
 			}
 		}
 	}

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

@@ -1851,7 +1851,14 @@ func Asmbelf(ctxt *Link, symo int64) {
 		if interpreter == "" {
 			switch ctxt.HeadType {
 			case objabi.Hlinux:
-				interpreter = thearch.Linuxdynld
+				if objabi.GOOS == "android" {
+					interpreter = thearch.Androiddynld
+					if interpreter == "" {
+						Exitf("ELF interpreter not set")
+					}
+				} else {
+					interpreter = thearch.Linuxdynld
+				}
 
 			case objabi.Hfreebsd:
 				interpreter = thearch.Freebsddynld

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

@@ -101,6 +101,7 @@ type Arch struct {
 	Minalign       int
 	Dwarfregsp     int
 	Dwarfreglr     int
+	Androiddynld   string
 	Linuxdynld     string
 	Freebsddynld   string
 	Netbsddynld    string