Fix JNI compiler for mips64

Fix assembly instructions in JNI stub to store the right size. Fix
callee-saves in JNI calling convention to reflect return registers.
Fix pointer size in mips64 assembler.

Change-Id: I4eccd1f56f62fa9119918192ec424e9073398bc7
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index f52f50e..b32e1f2 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -2214,10 +2214,8 @@
         InstructionSetHasGenericJniStub(instruction_set_)) {
       // Leaving this empty will trigger the generic JNI version
     } else {
-      if (instruction_set_ != kMips64) {  // Use generic JNI for Mips64 (temporarily).
-        compiled_method = compiler_->JniCompile(access_flags, method_idx, dex_file);
-        CHECK(compiled_method != nullptr);
-      }
+      compiled_method = compiler_->JniCompile(access_flags, method_idx, dex_file);
+      CHECK(compiled_method != nullptr);
     }
   } else if ((access_flags & kAccAbstract) != 0) {
     // Abstract methods don't have code.