perf evsel: Handle endianity in intval method

We were relying on the info in pevent, but since we have it in
perf_evsel, set up by the perf_session routine if read from a perf.data
file or by whoever creates the evsels, use it.

New 'perf test' entries will use it to parse locally generated events,
in a non perf.data centered workflow.

As well as use byteswap.h to get per arch optimized swap routines, like
other parts of perf (header, perf_evsel__parse_sample, symbol, etc)
already do.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-8tjuxk09mlsfmh7macgkxsip@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>