ART: Fix mips64 jni bugs.

For mips64, 32-bits loads can be zero extended or sign extended to
64-bits. The extension type must match the data type to be loaded.

Also re-enable mips64 generic jni testing.

Change-Id: I9cabaf80b4fde63d9868fccd74593b36d1c324e8
diff --git a/runtime/common_runtime_test.h b/runtime/common_runtime_test.h
index 9917378..34fdd8d 100644
--- a/runtime/common_runtime_test.h
+++ b/runtime/common_runtime_test.h
@@ -182,7 +182,7 @@
   }
 
 #define TEST_DISABLED_FOR_MIPS() \
-  if (kRuntimeISA == kMips || kRuntimeISA == kMips64) { \
+  if (kRuntimeISA == kMips) { \
     printf("WARNING: TEST DISABLED FOR MIPS\n"); \
     return; \
   }