scripts: make some scripts executable

Set the x bit to some scripts to make them directly executable.

Especially, scripts/checkdeclares.pl is not hooked by anyone.
It should be executable since it is tedious to type
'perl scripts/checkdeclares.pl'.

The original patch [1] set the x bit properly, but it was lost when
it was merged as commit 21917bded72c ("scripts: a new script for
checking duplicate struct declaration").

[1] https://lore.kernel.org/lkml/20210401110943.1010796-1-wanjiabing@vivo.com/

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

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>