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.

headers/os/arch/x86_64/arch_debugger.h

@@ -58,6 +58,8 @@ struct x86_64_debug_cpu_state {
 	uint64	rcx;
 	uint64	rbx;
 	uint64	rax;
+	uint64	vector;
+	uint64	error_code;
 	uint64	rip;
 	uint64	cs;
 	uint64	rflags;