Add -minimal mode to rustboot that skips emitting code that's broken or unneeded for rustc. Shrink rustc by 300kb. Back under 1mb.

src/boot/be/abi.ml

@@ -126,7 +126,8 @@ type abi =
                                -> Common.nabi
                                  -> Common.fixup (* grow_task *)
                                    -> bool       (* is_obj_fn *)
-                                     -> unit);
+                                     -> bool     (* minimal *)
+                                       -> unit);
 
     abi_emit_fn_epilogue: (Il.emitter -> unit);
 

src/boot/be/x86.ml

@@ -1236,6 +1236,7 @@ let fn_prologue
     (nabi:nabi)
     (grow_task_fixup:fixup)
     (is_obj_fn:bool)
+    (minimal:bool)
     : unit =
 
   let esi_n = word_n (h esi) in
@@ -1372,8 +1373,11 @@ let fn_prologue
       in
 
         (* "Full" frame size-check. *)
-        stack_growth_check e nabi grow_task_fixup
-          dynamic_frame_sz dynamic_grow_jmp restart_pc (h esi) (h edi);
+        match dynamic_grow_jmp with
+            None when minimal -> ()
+          | _ ->
+              stack_growth_check e nabi grow_task_fixup
+                dynamic_frame_sz dynamic_grow_jmp restart_pc (h esi) (h edi);
 
 
         (* Establish a frame, wherever we landed. *)