scripts: add generic syscallnr.sh

Like syscallhdr.sh and syscalltbl.sh, add a simple script to generate
the __NR_syscalls, which should not be exported to userspace.

This script is useful to replace arch/mips/kernel/syscalls/syscallnr.sh,
refactor arch/s390/kernel/syscalls/syscalltbl, and eliminate the code
surrounded by #ifdef __KERNEL__ / #endif from exported uapi/asm/unistd_*.h
files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>