x86/syscalls: Document the fact that syscalls 512-547 are a legacy mistake

Since this commit:

  6365b842

 ("x86/syscalls: Split the x32 syscalls into their own table")

there is no need for special x32-specific syscall numbers.  I forgot to
update the comments in syscall_64.tbl.  Add comments to make it clear to
future contributors that this range is a legacy wart.

Reported-by: Jessica Clarke <jrtc27@jrtc27.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/6c56fb4ddd18fc60a238eb4d867e4b3d97c6351e.1602471055.git.luto@kernel.org