Add system calls for x86

Adds an IDT entry for the SystemCall interrupt, modifies the exception
handler to call the system call handler, and creates a single syscall,
sys::write(), with the signature of Linux’s sys_write. The System V
AMD64 ABI is used for x86 because it’s simple to implement, since all
arguments are passed on registers.

Caveats:

*Since there’s no filesystem, only fd==stdout==1 works.

* Also, due to pointer casting difficulties (can’t figure out the
lifetime for buf_as_slice), sys::write ignores its buffer input and
writes a predetermined message.

* sys calls are confirmed to work _in-kernel_, but not tested in
user-space, since I haven’t gotten ELF execution to work on my system.