Although syscalls are done through SYSCALL and therefore don't actually
have an interrupt number, set it to 99 (the syscall vector on 32-bit)
in the iframe so that a syscall frame can be identified. Also added
vector/error_code to x86_64_debug_cpu_state for Debugger to use, not
sure why I didn't put them there in the first place.
Reused x86 arch_user_debugger.cpp, with a few minor changes to make
the code work for both 32 and 64 bit. Something isn't quite working
right, if a breakpoint is hit the kernel will hang. Other than that
everything appears to work correctly.
