perf bpf: Add syscall_exit() helper

perf bpf: Add syscall_exit() helper

So that we can hook to the syscalls:sys_exit_SYSCALL tracepoints in
addition to the syscalls:sys_enter_SYSCALL we hook using the
syscall_enter() helper.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-6qh8aph1jklyvdu7w89c0izc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>