perf top: Fix SIGBUS on sparc64

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

    commit 82396986032915c1572bfb74b224fcc2e4e8ba7c
    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>

tools/perf/util/evlist.h

@@ -28,7 +28,7 @@ struct perf_mmap {
 	int		 mask;
 	int		 refcnt;
 	unsigned int	 prev;
-	char		 event_copy[PERF_SAMPLE_MAX_SIZE];
+	char		 event_copy[PERF_SAMPLE_MAX_SIZE] __attribute__((aligned(8)));
 };
 
 struct perf_evlist {