Kernel: thread entrypoint fastcall ABI

Changing the way the kernel passes the argument to a thread entrypoint
to the fastcall ABI, so we no longer need the entrypoint to be a naked function.

kernel/src/i386/process_switch.rs

@@ -332,9 +332,9 @@ fn jump_to_entrypoint(ep: usize, userspace_stack_ptr: usize, arg: usize) -> ! {
         push $0     // Entrypoint
 
         // Clean up all registers. Also setup arguments.
-        mov eax, $2
+        mov ecx, $2
+        mov eax, 0
         mov ebx, 0
-        mov ecx, 0
         mov edx, 0
         mov ebp, 0
         mov edi, 0

libuser/src/syscalls.rs

@@ -145,7 +145,7 @@ pub fn exit_process() -> ! {
 }
 
 /// Creates a thread in the current process.
-pub fn create_thread(ip: extern fn() -> !, arg: usize, sp: *const u8, priority: u32, processor_id: u32) -> Result<Thread, KernelError> {
+pub fn create_thread(ip: extern "fastcall" fn(usize) -> !, arg: usize, sp: *const u8, priority: u32, processor_id: u32) -> Result<Thread, KernelError> {
     unsafe {
         let (out_handle, ..) = syscall(nr::CreateThread, ip as usize, arg, sp as _, priority as _, processor_id as _, 0)?;
         Ok(Thread(Handle::new(out_handle as _)))