perf evsel: Know if byte swap is needed

Instead of passing it around for parsing as an explicit parameter, will
help with reading tracepoint fields when not using a perf session or
pevent structure, i.e. for non perf.data centered workflows.

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-qa67ikv2sm49cwa7dyjhhp6g@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>