sneak an extra argument in by using rbx for the jump target

src/arch.rs

@@ -62,11 +62,11 @@ pub fn initialise_call_frame(stack: &mut Stack, init: uintptr_t, args: &[uintptr
     rbp: 0,
     rsp: sp as uintptr_t,
     ip: bootstrap as uintptr_t,
-    r12: init,
+    rbx: init,
     .. Registers::new()
   };
 
-  match into_fields!(regs { rdi, r13, r14, r15 } <- args.iter().cloned()) {
+  match into_fields!(regs { rdi, r12, r13, r14, r15 } <- args.iter().cloned()) {
     Some(mut args) => if args.next().is_some() {
       panic!("too many arguments")
     },

src/arch.s

@@ -84,11 +84,12 @@ lwt_bootstrap:
 ;; places the parameters in unrelated registers, and we frob them into place
 ;; out here, in assembly-land. below are the parameter registers in order,
 ;; along with the alternative register used in parentheses, if there is one.
-;; rdi, rsi (r13), rdx (r14), rcx (r15), r8, r9
-  mov rsi, r13
-  mov rdx, r14
-  mov rcx, r15
-  jmp r12
+;; rdi, rsi (r12), rdx (r13), rcx (r14), r8(r15), r9
+  mov rsi, r12
+  mov rdx, r13
+  mov rcx, r14
+  mov r8, r15
+  jmp rbx
 
 
 ;; Rust stores a stack limit at [fs:0x70]. These two functions set and retrieve