cmd: allow buildmode=c-shared for android/386

Update golang/go#9327.

Change-Id: Iab7dad31cf6b9f9347c3f34faebb67ecb38b17fc
Reviewed-on: https://go-review.googlesource.com/16701
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

build.go

@@ -351,7 +351,7 @@ func buildModeInit() {
 		} else {
 			switch platform {
 			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
-				"android/amd64", "android/arm":
+				"android/amd64", "android/arm", "android/386":
 				codegenArg = "-shared"
 			case "darwin/amd64":
 			default: