Reduce namespace pollution.

This leaves us with just the mspace stuff and three libdex functions to clean
up. We deliberately expose the JII API, and I don't think there's anything we
can really do about the art_..._from_code stuff (and at least that starts with
"art_").

Change-Id: I77e58e8330cd2afeb496642302dfe3311e68091a
diff --git a/src/compiler/codegen/arm/CodegenCommon.cc b/src/compiler/codegen/arm/CodegenCommon.cc
index 9b71b14..26c17ef 100644
--- a/src/compiler/codegen/arm/CodegenCommon.cc
+++ b/src/compiler/codegen/arm/CodegenCommon.cc
@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+namespace art {
+
 /*
  * This file contains codegen and support common to all supported
  * ARM variants.  It is included by:
@@ -399,3 +401,5 @@
     /* Mark all resources as being clobbered */
     barrier->defMask = -1;
 }
+
+}  // namespace art