Workaround KGDB issues on ARM by ignoring ARM EABI version higher than 5

To make KGDB working, it needs to understand kernel ELF image.
By default it is compiled using EABI_5, which is not supported
on the gdb-6. As a workaround, treat these images as EABI_2 because
they share a lot of things in common.

This workaround does not guarantee ALL funtionalities
to work.

Submitted by: Wojciech Macek <wma@semihalf.com>
Reviewed by: jhb
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4012