all: add GOOS=ios GOARCH=amd64 target for the ios simulator

The Go toolchain has supported the simulator for years, but always in
buildmode=c-archive which is intrinsically externally linked and PIE.

This CL moves that support from GOOS=darwin GOARCH=amd64 -tags=ios to
just GOOS=ios GOARCH=amd64 to match the change for iOS devices.

This change also forces external linking and defaults to buildmode=pie
to support Go binaries in the default buildmode to run on the simulator.

CL 255257 added the necessary support to the exec wrapper.

Updates #38485
Fixes #42100

Change-Id: I6e6ee0e8d421be53b31e3d403880e5b9b880d031
Reviewed-on: https://go-review.googlesource.com/c/go/+/263798
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Elias Naur <mail@eliasnaur.com>

src/cmd/dist/build.go

@@ -1557,6 +1557,7 @@ var cgoEnabled = map[string]bool{
 	"android/arm":     true,
 	"android/arm64":   true,
 	"ios/arm64":       true,
+	"ios/amd64":       true,
 	"js/wasm":         false,
 	"netbsd/386":      true,
 	"netbsd/amd64":    true,

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

@@ -1966,7 +1966,9 @@ func externalLinkingForced(p *Package) bool {
 		if cfg.BuildContext.GOARCH != "arm64" {
 			return true
 		}
-	case "darwin", "ios":
+	case "ios":
+		return true
+	case "darwin":
 		if cfg.BuildContext.GOARCH == "arm64" {
 			return true
 		}

src/cmd/go/internal/work/init.go

@@ -168,7 +168,10 @@ func buildModeInit() {
 			ldBuildmode = "pie"
 		case "windows":
 			ldBuildmode = "pie"
-		case "darwin", "ios":
+		case "ios":
+			codegenArg = "-shared"
+			ldBuildmode = "pie"
+		case "darwin":
 			switch cfg.Goarch {
 			case "arm64":
 				codegenArg = "-shared"

src/cmd/internal/sys/supported.go

@@ -88,6 +88,7 @@ func BuildModeSupported(compiler, buildmode, goos, goarch string) bool {
 			"android/amd64", "android/arm", "android/arm64", "android/386",
 			"freebsd/amd64",
 			"darwin/amd64", "darwin/arm64",
+			"ios/amd64", "ios/arm64",
 			"aix/ppc64",
 			"windows/386", "windows/amd64", "windows/arm":
 			return true

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

@@ -42,13 +42,7 @@ func (mode *BuildMode) Set(s string) error {
 		*mode = BuildModeExe
 	case "pie":
 		switch objabi.GOOS {
-		case "aix", "android", "linux", "windows":
-		case "darwin":
-			switch objabi.GOARCH {
-			case "amd64", "arm64":
-			default:
-				return badmode()
-			}
+		case "aix", "android", "linux", "windows", "darwin", "ios":
 		case "freebsd":
 			switch objabi.GOARCH {
 			case "amd64":

src/runtime/rt0_ios_amd64.s

@@ -0,0 +1,14 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+// internal linking executable entry point.
+// ios/amd64 only supports external linking.
+TEXT _rt0_amd64_ios(SB),NOSPLIT|NOFRAME,$0
+	UNDEF
+
+// library entry point.
+TEXT _rt0_amd64_ios_lib(SB),NOSPLIT|NOFRAME,$0
+	JMP	_rt0_amd64_darwin_lib(SB)