X64: Fix error in Win64 platform specific code in regexp stub.

Review URL: http://codereview.chromium.org/6626011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7062 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/code-stubs-x64.cc

@@ -2645,7 +2645,7 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   __ movq(arg2, rbx);
 
   // Argument 1: Subject string.
-#ifdef WIN64_
+#ifdef _WIN64
   __ movq(arg1, rdi);
 #else
   // Already there in AMD64 calling convention.