refactor: move syscall_init arch dispatch to arch/x86/sysenter_init.c

refactor: move syscall_init arch dispatch to arch/x86/sysenter_init.c

- Add arch_syscall_init() that registers INT 0x80 handler and calls x86_sysenter_init()
- syscall_init() now just calls arch_syscall_init() — zero #ifdef in syscall.c
- x86_sysenter_init() made static (internal to sysenter_init.c)
- syscall.c contains ZERO architecture-specific code or #ifdefs

20/20 smoke, cppcheck clean