Implemented the basis for syscalls

A software interrupt with vector 0x50 will cause a syscall to start executing.
The EAX register will hold the syscall_num.
Other registers and the stack can be used to hold further arguments.

kernel/interrupts/idt.h

@@ -70,6 +70,7 @@ extern "C" {
     extern void irs29 ();
     extern void irs30 ();
     extern void irs31 ();
+    extern void irs50();