x86_64: Set vector in iframe/debug state to 99 for syscalls.

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.