ARM: 6798/1: aout-core: zero thread debug registers in a.out core dump

The removal of the single-step emulation from ptrace on ARM means that
thread_struct no longer has software breakpoint fields in its debug
member.

This patch fixes the a.out core dump code so that the debug registers
are zeroed rather than trying to copy from non-existent fields.

Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>