cmd/dist, cmd/go: pass -arch for C compilation on Darwin

On Apple Silicon Mac, the C compiler has an annoying default
target selection, depending on the ancestor processes'
architecture. In particular, if the shell or IDE is x86, when
running "go build" even with a native ARM64 Go toolchain, the C
compiler defaults to x86, causing build failures. We pass "-arch"
flag explicitly to avoid this situation.

Fixes #43692.
Fixes #43476.
Updates golang/vscode-go#1087.

Change-Id: I80b6a116a114e11e273c6886e377a1cc969fa3f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/283812
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/cgo/gcc.go

@@ -1549,7 +1549,14 @@ func (p *Package) gccBaseCmd() []string {
 func (p *Package) gccMachine() []string {
 	switch goarch {
 	case "amd64":
+		if goos == "darwin" {
+			return []string{"-arch", "x86_64", "-m64"}
+		}
 		return []string{"-m64"}
+	case "arm64":
+		if goos == "darwin" {
+			return []string{"-arch", "arm64"}
+		}
 	case "386":
 		return []string{"-m32"}
 	case "arm":

src/cmd/go/internal/work/exec.go

@@ -2435,7 +2435,7 @@ func (b *Builder) fcExe() []string {
 func (b *Builder) compilerExe(envValue string, def string) []string {
 	compiler := strings.Fields(envValue)
 	if len(compiler) == 0 {
-		compiler = []string{def}
+		compiler = strings.Fields(def)
 	}
 	return compiler
 }
@@ -2581,7 +2581,14 @@ func (b *Builder) gccArchArgs() []string {
 	case "386":
 		return []string{"-m32"}
 	case "amd64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "x86_64", "-m64"}
+		}
 		return []string{"-m64"}
+	case "arm64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "arm64"}
+		}
 	case "arm":
 		return []string{"-marm"} // not thumb
 	case "s390x":

src/cmd/go/testdata/script/build_darwin_cc_arch.txt

@@ -0,0 +1,24 @@
+# Test that we pass -arch flag to C compiler on Darwin (issue 43692).
+
+[!darwin] skip
+[!cgo] skip
+
+# clear CC, in case user sets it
+env CC=
+
+env CGO_ENABLED=1
+
+env GOARCH=amd64
+go build -n -x c.go
+stderr 'clang.*-arch x86_64'
+
+env GOARCH=arm64
+go build -n -x c.go
+stderr 'clang.*-arch arm64'
+
+-- c.go --
+package main
+
+import "C"
+
+func main() {}

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

@@ -1749,12 +1749,19 @@ func hostlinkArchArgs(arch *sys.Arch) []string {
 	switch arch.Family {
 	case sys.I386:
 		return []string{"-m32"}
-	case sys.AMD64, sys.S390X:
+	case sys.AMD64:
+		if objabi.GOOS == "darwin" {
+			return []string{"-arch", "x86_64", "-m64"}
+		}
+		return []string{"-m64"}
+	case sys.S390X:
 		return []string{"-m64"}
 	case sys.ARM:
 		return []string{"-marm"}
 	case sys.ARM64:
-		// nothing needed
+		if objabi.GOOS == "darwin" {
+			return []string{"-arch", "arm64"}
+		}
 	case sys.MIPS64:
 		return []string{"-mabi=64"}
 	case sys.MIPS: