perf trace: Allow specifying which syscalls to trace

Similar to -e in strace, i.e. a comma separated list of syscall names
to trace.

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-5zku7q5wug3103k1dzn3yy63@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-trace.txt

@@ -26,6 +26,10 @@ OPTIONS
 --all-cpus::
         System-wide collection from all CPUs.
 
+-e::
+--expr::
+	List of events to show, currently only syscall names.
+
 -p::
 --pid=::
 	Record events on existing process ID (comma separated list).