scripts: add generic syscalltbl.sh

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

The syscall table has the same format for all architectures. Each line
has up to 5 fields; syscall number, ABI, syscall name, native entry
point, and compat entry point. The syscall table is processed by
syscalltbl.sh script into header files.

Despite the same pattern, scripts are maintained per architecture,
which results in code duplication and bad maintainability.

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

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

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

__SYSCALL_WITH_COMPAT should be defined as follows:

32-bit kernel:
  #define __SYSCALL_WITH_COMPAT(nr, native, compat) __SYSCALL(nr, native)

64-bit kernel:

  #define __SYSCALL_WITH_COMPAT(nr, native, compat) __SYSCALL(nr, compat)

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