perf trace: Add support for syscalls vs raw_syscalls

Older kernels (e.g., RHEL6) do system call tracing via
syscalls:sys_{enter,exit} rather than raw_syscalls. Update perf-trace to
detect lack of raw_syscalls support and try syscalls.

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1386211302-31303-2-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>