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/compiler/llvm/llvm_compilation_unit.cc b/compiler/llvm/llvm_compilation_unit.cc
index fe60959..78bdb4d 100644
--- a/compiler/llvm/llvm_compilation_unit.cc
+++ b/compiler/llvm/llvm_compilation_unit.cc
@@ -143,7 +143,7 @@
 
 static std::string DumpDirectory() {
   if (kIsTargetBuild) {
-    return GetDalvikCacheOrDie(GetAndroidData());
+    return GetDalvikCacheOrDie("llvm-dump");
   }
   return "/tmp";
 }