powerpc/watchpoint: Support for 8xx in ptrace-hwbreak.c selftest

On the 8xx, signals are generated after executing the instruction. So
no need to manually single-step on 8xx. Also, 8xx __set_dabr()
currently ignores length and hardcodes the length to 8 bytes. So all
unaligned and 512 byte testcase will fail on 8xx. Ignore those
testcases on 8xx.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191017093204.7511-8-ravi.bangoria@linux.ibm.com