gdb: do not allow native gdb on SuperH architecture

The gdb debugger does not have support for running as the native
debugger on the SuperH architecture:

 configure: error: "*** Gdb does not support native target sh4-unknown-linux-gnu"

See also http://lists.debian.org/debian-superh/2010/04/msg00000.html

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>