scripts: add check_extable.sh script.

This shell script can be used to sanity check the __ex_table section on an
object file, making sure the relocations in there are pointing to valid
executable sections.  If it finds some suspicious relocations, it'll use
addr2line to try and dump where this is coming from.

This works best with CONFIG_DEBUG_INFO.

Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>