cmd/go, cmd/link: enable -buildmode=c-shared on linux/386

All the heavy lifting was done by Michael Hudson-Doyle.

Change-Id: I176f15581055078854c2ad9a5807c4dcf0f8d8c5
Reviewed-on: https://go-review.googlesource.com/17074Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/build.go

@@ -350,7 +350,7 @@ func buildModeInit() {
 			codegenArg = "-fPIC"
 		} else {
 			switch platform {
-			case "linux/amd64", "linux/arm", "linux/arm64",
+			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
 				"android/amd64", "android/arm":
 				codegenArg = "-shared"
 			case "darwin/amd64":

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

@@ -318,7 +318,9 @@ func (mode *BuildMode) Set(s string) error {
 		}
 		*mode = BuildmodeCArchive
 	case "c-shared":
-		if goarch != "amd64" && goarch != "arm" && goarch != "arm64" {
+		switch goarch {
+		case "386", "amd64", "arm", "arm64":
+		default:
 			return badmode()
 		}
 		*mode = BuildmodeCShared
@@ -1696,7 +1698,8 @@ func stkcheck(up *Chain, depth int) int {
 		// should never be called directly.
 		// only diagnose the direct caller.
 		// TODO(mwhudson): actually think about this.
-		if depth == 1 && s.Type != obj.SXREF && !DynlinkingGo() && Buildmode != BuildmodePIE {
+		if depth == 1 && s.Type != obj.SXREF && !DynlinkingGo() &&
+			Buildmode != BuildmodePIE && Buildmode != BuildmodeCShared {
 			Diag("call to external function %s", s.Name)
 		}
 		return -1

src/cmd/link/internal/x86/asm.go

@@ -51,7 +51,7 @@ func addcall(ctxt *ld.Link, s *ld.LSym, t *ld.LSym) {
 }
 
 func gentext() {
-	if !ld.DynlinkingGo() && ld.Buildmode != ld.BuildmodePIE {
+	if !ld.DynlinkingGo() && ld.Buildmode != ld.BuildmodePIE && ld.Buildmode != ld.BuildmodeCShared {
 		return
 	}