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/Codegen.h b/src/compiler/codegen/arm/Codegen.h
index 24e2b3e..8d3c425 100644
--- a/src/compiler/codegen/arm/Codegen.h
+++ b/src/compiler/codegen/arm/Codegen.h
@@ -24,6 +24,8 @@
 
 #include "../../CompilerIR.h"
 
+namespace art {
+
 #if defined(_CODEGEN_C)
 /*
  * loadConstant() sometimes needs to add a small imm to a pre-existing constant
@@ -50,3 +52,5 @@
 
 extern ArmLIR* oatRegCopyNoInsert(CompilationUnit* cUnit, int rDest,
                                           int rSrc);
+
+}  // namespace art