Move 'event_copy' from tools/perf's mmap to libperf's perf_mmap struct.

Committer notes:

Add linux/compiler.h as we need it for '__aligned'.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lore.kernel.org/lkml/20190913132355.21634-18-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>