checksyscalls: Use arch/x86/syscalls/syscall_32.tbl as source

Use the new arch/x86/syscalls/syscall_32.tbl file as source instead of
arch/x86/include/asm/unistd_32.h.

Cc: Michal Marek <mmarek@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>