Bug 1460645 Avoid the thread_local keyword for MinGW and instead use __thread r=froydnj

MozReview-Commit-ID: 5RPwQVatThs

--HG--
extra : rebase_source : 0ad9a69f51529c6a35dc76555e4cb5d37a087a28

mfbt/ThreadLocal.h

@@ -237,7 +237,7 @@ ThreadLocal<T, Storage>::set(const T aValue)
   }
 }
 
-#if defined(XP_WIN) || defined(MACOSX_HAS_THREAD_LOCAL)
+#if (defined(XP_WIN) || defined(MACOSX_HAS_THREAD_LOCAL)) && !defined(__MINGW32__)
 #define MOZ_THREAD_LOCAL(TYPE) thread_local mozilla::detail::ThreadLocal<TYPE, mozilla::detail::ThreadLocalNativeStorage>
 #elif defined(HAVE_THREAD_TLS_KEYWORD)
 #define MOZ_THREAD_LOCAL(TYPE) __thread mozilla::detail::ThreadLocal<TYPE, mozilla::detail::ThreadLocalNativeStorage>