samples/trace_event: Add '__rel_loc' using sample event

samples/trace_event: Add '__rel_loc' using sample event

Add '__rel_loc' using sample event for testing.
User can use this for testing purpose. There is
no reason to use this macro from the kernel.

Link: https://lkml.kernel.org/r/163757343050.510314.2876529802471645178.stgit@devnote2
Cc: Beau Belgrave <beaub@linux.microsoft.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>