gcc: obey EXTERNAL_ENABLED in the bbappend

Signed-off-by: Christopher Larson <kergoth@gmail.com>

core/recipes-devtools/gcc/gcc_%.bbappend

@@ -1,7 +1,7 @@
 # FIXME: resolve gcc version mismatch issues with ${libdir}/gcc/*/<version>
 # filesystem paths
 python () {
-    if d.getVar('TCMODE', True).startswith('external'):
+    if d.getVar('EXTERNAL_ENABLED'):
         # unwind.h will come from libgcc-external, we don't want to try to pull it
         # from the cross area of the sysroot
         inst = d.getVar('do_install', False).splitlines()