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

arch/x86/entry/syscalls/syscall_64.tbl

@@ -363,10 +363,10 @@
 439	common	faccessat2		sys_faccessat2
 
 #
-# x32-specific system call numbers start at 512 to avoid cache impact
-# for native 64-bit operation. The __x32_compat_sys stubs are created
-# on-the-fly for compat_sys_*() compatibility system calls if X86_X32
-# is defined.
+# Due to a historical design error, certain syscalls are numbered differently
+# in x32 as compared to native x86_64.  These syscalls have numbers 512-547.
+# Do not add new syscalls to this range.  Numbers 548 and above are available
+# for non-x32 use.
 #
 512	x32	rt_sigaction		compat_sys_rt_sigaction
 513	x32	rt_sigreturn		compat_sys_x32_rt_sigreturn
@@ -404,3 +404,5 @@
 545	x32	execveat		compat_sys_execveat
 546	x32	preadv2			compat_sys_preadv64v2
 547	x32	pwritev2		compat_sys_pwritev64v2
+# This is the end of the legacy x32 range.  Numbers 548 and above are
+# not special and are not to be used for x32-specific syscalls.