perf top: Fix SIGBUS on sparc64

perf-top is terminating due to SIGBUS on sparc64. git bisect points to:

    commit 82396986
    Author: Arnaldo Carvalho de Melo <acme@redhat.com>
    Date:   Mon Sep 8 13:26:35 2014 -0300

        perf evlist: Refcount mmaps

        We need to know how many fds are using a perf mmap via
        PERF_EVENT_IOC_SET_OUTPUT, so that we can know when to ditch an mmap,
        refcount it.

This commit added 'int refcnt' to struct perf_mmap and the addition makes the
event_copy element no longer 8-byte aligned.

Fix by adding __attribute__((aligned(8))) to the event_copy struct
member.

Signed-off-by: David Ahern <david.ahern@oracle.com>
Link: http://lkml.kernel.org/r/1424304198-92028-1-git-send-email-david.ahern@oracle.com


[ Switched from 'int pad;' to using __attribute__, David tested/acked that ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>