Add partial support for building with MingW64 GCC 4.8-SEH.

Error handling works, C++ interoperability generally works.
C++ destructors in libs compiled with G++ cause trouble (ok with MSVC).

src/lj_ccallback.c

@@ -286,9 +286,9 @@ void lj_ccallback_mcode_free(CTState *cts)
 /* Windows/x64 argument registers are strictly positional (use ngpr). */
 #define CALLBACK_HANDLE_REGARG \
   if (isfp) { \
-    if (ngpr < 4) { sp = &cts->cb.fpr[ngpr++]; nfpr = ngpr; goto done; } \
+    if (ngpr < maxgpr) { sp = &cts->cb.fpr[ngpr++]; UNUSED(nfpr); goto done; } \
   } else { \
-    if (ngpr < 4) { sp = &cts->cb.gpr[ngpr++]; goto done; } \
+    if (ngpr < maxgpr) { sp = &cts->cb.gpr[ngpr++]; goto done; } \
   }
 
 #elif LJ_TARGET_X64