Use instruction specific dalvik cache dirs.

- All oat & art files are now placed under /data/dalvik-cache/<isa>/.
- GetDalvikCacheOrDie now requires a mandatory subdirectory argument,
  and is implicitly rooted under /data/.
- Added helper methods to convert InstructionSet enums into strings
  and vice versa.

(cherry picked from commit 2974bc3d8a5d161d449dd66826d668d87bdc3cbe)

Change-Id: Ic7986938e6a7091a2af675ebafec768f7b5fb8cd
diff --git a/runtime/instruction_set.h b/runtime/instruction_set.h
index c746e06..4bc35a7 100644
--- a/runtime/instruction_set.h
+++ b/runtime/instruction_set.h
@@ -35,6 +35,9 @@
 };
 std::ostream& operator<<(std::ostream& os, const InstructionSet& rhs);
 
+const char* GetInstructionSetString(const InstructionSet isa);
+InstructionSet GetInstructionSetFromString(const char* instruction_set);
+
 size_t GetInstructionSetPointerSize(InstructionSet isa);
 size_t GetInstructionSetAlignment(InstructionSet isa);
 bool Is64BitInstructionSet(InstructionSet isa);