hw_breakpoint: Add step_needed event attribute

Architecture like ARM64 currently allows to use default hw breakpoint
single step handler only to perf. However, some other users like few
systemtap tests or kernel test in
samples/hw_breakpoint/data_breakpoint.c can also work with default step
handler implementation.

Therefore, this patch introduces a flag 'step_needed' in struct
hw_perf_event, so that arch specific code(specially on arm64) can make a
decision to enable single stepping.

Any architecture which is not using this field will not have any
side effect.

Signed-off-by: Pratyush Anand <panand@redhat.com>