perf trace: Consider syscall aliases too

When trying to trace the 'umount' syscall on x86_64 I noticed that it
was failing:

  # trace -e umount umount /mnt
  event syntax error: 'umount'
                       \___ parser error
  Run 'perf list' for a list of valid events

   Usage: perf trace [<options>] [<command>]
      or: perf trace [<options>] -- <command> [<options>]
      or: perf trace record [<options>] [<command>]
      or: perf trace record [<options>] -- <command> [<options>]

      -e, --event <event>   event/syscall selector. use 'perf list' to list available events
  #

This is because in the x86-64 we have it just as 'umount2':

  $ grep umount arch/x86/entry/syscalls/syscall_64.tbl
  166	common	umount2			__x64_sys_umount
  $

So if the syscall name fails, try fallbacking to looking at the aliases
we have in the syscall_fmts table to then re-lookup, now:

  # trace -e umount umount -f /mnt
  umount: /mnt: not mounted.
     1.759 ( 0.004 ms): umount/18365 umount2(name: 0x55fbfcbc4480, flags: 1) = -1 EINVAL Invalid argument
  #

Time to beautify the flags arg :-)

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Benjamin Peterson <benjamin@python.org>
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-ukweodgzbmjd25lfkgryeft1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>