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/compiler/utils/mips64/assembler_mips64.h b/compiler/utils/mips64/assembler_mips64.h
index b7f6a9e..2d7c661 100644
--- a/compiler/utils/mips64/assembler_mips64.h
+++ b/compiler/utils/mips64/assembler_mips64.h
@@ -36,6 +36,7 @@
   kLoadSignedHalfword,
   kLoadUnsignedHalfword,
   kLoadWord,
+  kLoadUnsignedWord,
   kLoadDoubleword
 };
 
@@ -85,6 +86,7 @@
   void Ld(GpuRegister rt, GpuRegister rs, uint16_t imm16);
   void Lbu(GpuRegister rt, GpuRegister rs, uint16_t imm16);
   void Lhu(GpuRegister rt, GpuRegister rs, uint16_t imm16);
+  void Lwu(GpuRegister rt, GpuRegister rs, uint16_t imm16);
   void Lui(GpuRegister rt, uint16_t imm16);
   void Mfhi(GpuRegister rd);
   void Mflo(GpuRegister rd);