perf trace: Allow overiding the formatting of syscall fields

perf trace: Allow overiding the formatting of syscall fields

The mmap syscalls, for instance, don't have the FORMAT_IS_POINTER for
its pointer arguments, override it.

This also paves the way for more specialized argument beautifiers, like
for mmap's prot and flags arguments.

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-mm864hvhrpt39muxmmbtjasz@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>