Improve syscalls for new task switching

There are a lot of under the hood changes here:
- Move syscalls to be a dispatch table, defined by syscalls.inc
- Don't need a full process state (push_all) in syscalls now
- In push_all, define REGS instead of using offsets
- Save TWO stack pointers as well as current saved stack pointer in TCB:
  - rsp0 is the base of the kernel stack for interrupts
  - rsp3 is the saved user stack from cpu_data
- Update syscall numbers in nulldrv
- Some asm-debugging enhancements to the gdb script
- fork() still not working

src/kernel/syscall.h

@@ -6,20 +6,12 @@ struct cpu_state;
 
 enum class syscall : uint64_t
 {
-	noop           = 0x0000,
-	debug          = 0x0001,
-	message        = 0x0002,
-	pause          = 0x0003,
-	sleep          = 0x0004,
-	getpid         = 0x0005,
-	send           = 0x0006,
-	receive        = 0x0007,
-	fork           = 0x0008,
-	exit           = 0x0009,
+#define SYSCALL(name, nargs) name ,
+#include "syscalls.inc"
+#undef SYSCALL
 
-	last_syscall
+	COUNT
 };
 
 void syscall_enable();
-void syscall_dispatch(cpu_state *);
-
+extern "C" void syscall_invalid(uint64_t call);