Fix compilation with recent MinGW64 versions.

src/ia32/regexp-macro-assembler-ia32.cc

Index: src/ia32/regexp-macro-assembler-ia32.cc
diff --git a/src/ia32/regexp-macro-assembler-ia32.cc b/src/ia32/regexp-macro-assembler-ia32.cc
index dfcc8695675617775e1422e921118f76b70b70bc..7d9e2f70527f6994470aa53a96dfce3c90a4c851 100644
--- a/src/ia32/regexp-macro-assembler-ia32.cc
+++ b/src/ia32/regexp-macro-assembler-ia32.cc
@@ -711,7 +711,7 @@ Handle<HeapObject> RegExpMacroAssemblerIA32::GetCode(Handle<String> source) {
   // position registers.
   __ mov(Operand(ebp, kInputStartMinusOne), eax);
 
-#ifdef WIN32
+#if V8_OS_WIN
   // Ensure that we write to each stack page, in order. Skipping a page
   // on Windows can cause segmentation faults. Assuming page size is 4k.
   const int kPageSize = 4096;
@@ -721,7 +721,7 @@ Handle<HeapObject> RegExpMacroAssemblerIA32::GetCode(Handle<String> source) {
       i += kRegistersPerPage) {
     __ mov(register_location(i), eax);  // One write every page.
   }
-#endif  // WIN32
+#endif  // V8_OS_WIN
 
   Label load_char_start_regexp, start_regexp;
   // Load newline if index is at start, previous character otherwise.