cmd/compile, cmd/go: generate position independent code with -buildmode=c-shared on arm

All the code was there to do this, it just wasn't hooked up.

Fixes #10914

Change-Id: Ide8f9bbe50fecb5d11cd579915ee98d4c7efe403
Reviewed-on: https://go-review.googlesource.com/14142Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -237,9 +237,11 @@ func Main() {
 	obj.Flagcount("y", "debug declarations in canned imports (with -d)", &Debug['y'])
 	var flag_shared int
 	var flag_dynlink bool
+	if Thearch.Thechar == '6' || Thearch.Thechar == '5' {
+		obj.Flagcount("shared", "generate code that can be linked into a shared library", &flag_shared)
+	}
 	if Thearch.Thechar == '6' {
 		obj.Flagcount("largemodel", "generate code that assumes a large memory model", &flag_largemodel)
-		obj.Flagcount("shared", "generate code that can be linked into a shared library", &flag_shared)
 		flag.BoolVar(&flag_dynlink, "dynlink", false, "support references to Go symbols defined in other shared libraries")
 	}
 	obj.Flagstr("cpuprofile", "write cpu profile to `file`", &cpuprofile)

src/cmd/go/build.go

@@ -344,12 +344,9 @@ func buildModeInit() {
 			codegenArg = "-fPIC"
 		} else {
 			switch platform {
-			case "linux/amd64":
+			case "linux/amd64", "linux/arm", "android/arm":
 				codegenArg = "-shared"
-			case "linux/arm":
-				buildAsmflags = append(buildAsmflags, "-shared")
 			case "darwin/amd64":
-			case "android/arm":
 			default:
 				fatalf("-buildmode=c-shared not supported on %s\n", platform)
 			}