gcc: don't pull unwind.h from cross sysroot

libgcc-external provides this.

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

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

@@ -0,0 +1,8 @@
+# unwind.h will come from libgcc-external, we don't want to try to pull it
+# from the cross area of the sysroot
+python () {
+    if d.getVar('TCMODE', True).startswith('external-sourcery'):
+        inst = d.getVar('do_install', False).splitlines()
+        inst = filter(lambda l: not ('unwind.h' in l and '${STAGING_LIBDIR_NATIVE}' in l), inst)
+        d.setVar('do_install', '\n'.join(inst))
+}