ART: Make InstructionSet an enum class and add kLast.

Adding InstructionSet::kLast shall make it easier to encode
the InstructionSet in fewer bits using BitField<>. However,
introducing `kLast` into the `art` namespace is not a good
idea, so we change the InstructionSet to an enum class.
This also uncovered a case of InstructionSet::kNone being
erroneously used instead of vixl32::Condition::None(), so
it's good to remove `kNone` from the `art` namespace.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I6fa6168dfba4ed6da86d021a69c80224f09997a6
diff --git a/runtime/common_runtime_test.cc b/runtime/common_runtime_test.cc
index f15acf9..0515bc9 100644
--- a/runtime/common_runtime_test.cc
+++ b/runtime/common_runtime_test.cc
@@ -334,26 +334,26 @@
 
 std::string CommonRuntimeTestImpl::GetAndroidTargetToolsDir(InstructionSet isa) {
   switch (isa) {
-    case kArm:
-    case kThumb2:
+    case InstructionSet::kArm:
+    case InstructionSet::kThumb2:
       return GetAndroidToolsDir("prebuilts/gcc/linux-x86/arm",
                                 "arm-linux-androideabi",
                                 "arm-linux-androideabi");
-    case kArm64:
+    case InstructionSet::kArm64:
       return GetAndroidToolsDir("prebuilts/gcc/linux-x86/aarch64",
                                 "aarch64-linux-android",
                                 "aarch64-linux-android");
-    case kX86:
-    case kX86_64:
+    case InstructionSet::kX86:
+    case InstructionSet::kX86_64:
       return GetAndroidToolsDir("prebuilts/gcc/linux-x86/x86",
                                 "x86_64-linux-android",
                                 "x86_64-linux-android");
-    case kMips:
-    case kMips64:
+    case InstructionSet::kMips:
+    case InstructionSet::kMips64:
       return GetAndroidToolsDir("prebuilts/gcc/linux-x86/mips",
                                 "mips64el-linux-android",
                                 "mips64el-linux-android");
-    case kNone:
+    case InstructionSet::kNone:
       break;
   }
   ADD_FAILURE() << "Invalid isa " << isa;