Fix compilation with recent MinGW64 versions.

src/x64/regexp-macro-assembler-x64.cc

Index: src/x64/regexp-macro-assembler-x64.cc
diff --git a/src/x64/regexp-macro-assembler-x64.cc b/src/x64/regexp-macro-assembler-x64.cc
index dcd317c666edbd122e65de85c392d076aa1329ce..49d9de65ab96229088b88b0c3e919631f3351283 100644
--- a/src/x64/regexp-macro-assembler-x64.cc
+++ b/src/x64/regexp-macro-assembler-x64.cc
@@ -761,7 +761,7 @@ Handle<HeapObject> RegExpMacroAssemblerX64::GetCode(Handle<String> source) {
   // position registers.
   __ movq(Operand(rbp, kInputStartMinusOne), rax);
 
-#ifdef WIN32
+#if V8_OS_WIN
   // Ensure that we have written to each stack page, in order. Skipping a page
   // on Windows can cause segmentation faults. Assuming page size is 4k.
   const int kPageSize = 4096;
@@ -771,7 +771,7 @@ Handle<HeapObject> RegExpMacroAssemblerX64::GetCode(Handle<String> source) {
       i += kRegistersPerPage) {
     __ movq(register_location(i), rax);  // One write every page.
   }
-#endif  // WIN32
+#endif  // V8_OS_WIN
 
   // Initialize code object pointer.
   __ Move(code_object_pointer(), masm_.CodeObject());