scripts: check duplicated syscall number in syscall table

Currently, syscall{hdr,tbl}.sh sorts the entire syscall table, but you
can assume it is already sorted by the syscall number.

The generated syscall table does not work if the same syscall number
appears twice. Check it in the script.

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

scripts: add generic syscallhdr.sh

Most of architectures generate syscall headers at the compile time
in a similar way.

As of v5.11-rc1, 12 architectures duplicate similar shell scripts:

$ find arch -name syscallhdr.sh | sort
arch/alpha/kernel/syscalls/syscallhdr.sh
arch/arm/tools/syscallhdr.sh
arch/ia64/kernel/syscalls/syscallhdr.sh
arch/m68k/kernel/syscalls/syscallhdr.sh
arch/microblaze/kernel/syscalls/syscallhdr.sh
arch/mips/kernel/syscalls/syscallhdr.sh
arch/parisc/kernel/syscalls/syscallhdr.sh
arch/powerpc/kernel/syscalls/syscallhdr.sh
arch/sh/kernel/syscalls/syscallhdr.sh
arch/sparc/kernel/syscalls/syscallhdr.sh
arch/x86/entry/syscalls/syscallhdr.sh
arch/xtensa/kernel/syscalls/syscallhdr.sh

My goal is to unify them into scripts/syscallhdr.sh.

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