perf trace: Allow printing syscall return values in hex

event_format->flags has a FIELD_IS_POINTER, but it is not set for
the sys_exit 'ret' field in syscalls like mmap, so we need a way to
ask for hex printing for pointer returns and keep things like 'read'
returns printing in decimal.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
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-lfuveegw4od1t08n7bsmonrm@git.kernel.org


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