cmd: fix handling of (multiple) tags

The gomobile tool mishandled build tags in two ways, first by
ignoring tags for iOS, second by passing multiple tags along to
the go tool incorrectly. This CL fixes both.

Fixes golang/go#18523
Fixes golang/go#18515

Change-Id: I28a49c1e23670adb085617d9f5fb5cd5e22a4b65
Reviewed-on: https://go-review.googlesource.com/34955
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/SeqTest.java

@@ -581,4 +581,8 @@ public class SeqTest extends InstrumentationTestCase {
   public void testSIGPIPE() {
     Testpkg.testSIGPIPE();
   }
+
+  public void testTags() {
+    assertEquals("Constant from a tagged file", 42, Testpkg.TaggedConst);
+  }
 }