Fix __ImageBase issue and revert FLS usage

Author: DinoV

Include/internal/pycore_mimalloc.h

@@ -17,11 +17,6 @@
 // CPython never uses mimalloc's C++ operator new APIs, so don't define a weak
 // std::get_new_handler() (_ZSt15get_new_handlerv) in libpython.
 #  define MI_NO_CXX_NEW_HANDLER 1
-#  ifdef MS_WINDOWS
-// Initialize from a CRT initializer and use FLS for thread cleanup instead
-// of TLS callbacks, which run during DLL attach before the CRT is set up.
-#    define MI_WIN_INIT_USE_FLS 1
-#  endif
 #  ifdef Py_GIL_DISABLED
 #    define MI_PRIM_THREAD_ID   _Py_ThreadId
 #    define MI_CPYTHON 1

Objects/mimalloc/prim/windows/prim.c

@@ -854,7 +854,13 @@ static void NTAPI mi_win_main(PVOID module, DWORD reason, LPVOID reserved) {
     return ((imageNtHeaders->FileHeader.Characteristics & IMAGE_FILE_DLL) == IMAGE_FILE_DLL);
   }
 
-  mi_decl_externc IMAGE_DOS_HEADER __ImageBase;   // supplied by the linker
+#if defined(__cplusplus)
+extern "C" {
+#endif
+  extern IMAGE_DOS_HEADER __ImageBase;   // supplied by the linker
+#if defined(__cplusplus)
+}
+#endif
 
   static bool mi_current_module_is_dll(void) {
     // HMODULE mod = NULL;