Support 'syscall' instruction and syscalls with 0-6 arguments

[ryuukumar]

Couple of improvements:

• change the syscall numbers to group them together by categories
• separate handler types for syscalls with 0 to 6 arguments, so that there are no 'unused' arguments, and syscalls like mmap can eventually be implemented
• use the syscall instruction from user mode
  • the legcay int 0x80 path is preserved for use by kernel processes + compatibility