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.h b/runtime/common_runtime_test.h
index e2131f1..5be8d5b 100644
--- a/runtime/common_runtime_test.h
+++ b/runtime/common_runtime_test.h
@@ -253,13 +253,13 @@
   }
 
 #define TEST_DISABLED_FOR_MIPS() \
-  if (kRuntimeISA == kMips) { \
+  if (kRuntimeISA == InstructionSet::kMips) { \
     printf("WARNING: TEST DISABLED FOR MIPS\n"); \
     return; \
   }
 
 #define TEST_DISABLED_FOR_X86() \
-  if (kRuntimeISA == kX86) { \
+  if (kRuntimeISA == InstructionSet::kX86) { \
     printf("WARNING: TEST DISABLED FOR X86\n"); \
     return; \
   }