MIPS: Fix bug in StringConstructCode builtin.

BUG=

TEST=webkit/dfg-to-string-bad-toString, webkit/dfg-to-string-bad-valueOf,
webkit/dfg-to-string-int-or-string, webkit/dfg-to-string-side-effect,
webkit/dfg-to-string-bad-toString, webkit/dfg-to-string-bad-valueOf,
webkit/dfg-to-string-toString-in-string, webkit/dfg-to-string-int,
webkit/dfg-to-string-valueOf-in-string

Review URL: https://codereview.chromium.org/18668004
Patch from Dusan Milosavljevic <Dusan.Milosavljevic@rt-rk.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15489 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/builtins-mips.cc

@@ -273,7 +273,7 @@ void Builtins::Generate_StringConstructCode(MacroAssembler* masm) {
   __ IncrementCounter(counters->string_ctor_conversions(), 1, a3, t0);
   {
     FrameScope scope(masm, StackFrame::INTERNAL);
-    __ push(v0);
+    __ push(a0);
     __ InvokeBuiltin(Builtins::TO_STRING, CALL_FUNCTION);
   }
   __ pop(function);