Fix code generation for String.<init> on x64.

The ArtMethod is a 64bit pointer so should be loaded with movq.

Change-Id: I80803046a9144776d7f069e8baee61e39ae289d5
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index ffd8c42..f6eb1d5 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -786,7 +786,7 @@
   switch (invoke->GetMethodLoadKind()) {
     case HInvokeStaticOrDirect::MethodLoadKind::kStringInit:
       // temp = thread->string_init_entrypoint
-      __ gs()->movl(temp.AsRegister<CpuRegister>(),
+      __ gs()->movq(temp.AsRegister<CpuRegister>(),
                     Address::Absolute(invoke->GetStringInitOffset(), /* no_rip */ true));
       break;
     case HInvokeStaticOrDirect::MethodLoadKind::kRecursive: