Add support for __FreeBSD_kernel__ kernel

Add support for kfreebsd-i386 and kfreebsd-amd64 Debian arch with
thanks to Pierre Chifflier <chifflier@cpe.fr>.

libtcc.c

@@ -943,6 +943,9 @@ LIBTCCAPI TCCState *tcc_new(void)
     tcc_define_symbol(s, "__FreeBSD__", str( __FreeBSD__));
 #undef str
 #endif
+#if defined(__FreeBSD_kernel__)
+    tcc_define_symbol(s, "__FreeBSD_kernel__", NULL);
+#endif
 #if defined(__linux)
     tcc_define_symbol(s, "__linux__", NULL);
     tcc_define_symbol(s, "__linux", NULL);