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/prebuilt_tools_test.cc b/runtime/prebuilt_tools_test.cc
index c2b34c8..6fa9b34 100644
--- a/runtime/prebuilt_tools_test.cc
+++ b/runtime/prebuilt_tools_test.cc
@@ -50,7 +50,7 @@
 
 TEST_F(PrebuiltToolsTest, CheckTargetTools) {
   // Other prebuilts are missing from the build server's repo manifest.
-  InstructionSet isas[] = { kThumb2 };  // NOLINT
+  InstructionSet isas[] = { InstructionSet::kThumb2 };  // NOLINT
   for (InstructionSet isa : isas) {
     std::string tools_dir = GetAndroidTargetToolsDir(isa);
     if (tools_dir.empty()) {