cmd/dist, cmd/link: force external linking for shared libs on arm

Also run testcshared.

Fixes #12425

Change-Id: I5baea8d772d3462f945aab96260b4197dbb20c0a
Reviewed-on: https://go-review.googlesource.com/14143Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -570,7 +570,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 	case "c-shared":
 		// TODO(hyangah): add linux-386.
 		switch pair {
-		case "linux-amd64", "darwin-amd64", "android-arm":
+		case "linux-amd64", "darwin-amd64", "android-arm", "linux-arm":
 			return true
 		}
 		return false

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

@@ -86,6 +86,10 @@ func archinit() {
 		ld.Linkmode = ld.LinkInternal
 	}
 
+	if ld.Buildmode == ld.BuildmodeCArchive || ld.Buildmode == ld.BuildmodeCShared || ld.DynlinkingGo() {
+		ld.Linkmode = ld.LinkExternal
+	}
+
 	switch ld.HEADTYPE {
 	default:
 		if ld.Linkmode == ld.LinkAuto {