auto merge of #5646 : Aatch/rust/unwind-fix, r=brson

This fixes issue #5641

Author: bors

src/rt/rust_unwind.h

@@ -19,10 +19,16 @@
 #include "rust_globals.h"
 #include "rust_task.h"
 #include "rust_sched_loop.h"
-#include "rust_unwind.h"
 #include "rust_upcall.h"
 #include "rust_util.h"
 
+//Unwinding ABI declarations.
+typedef int _Unwind_Reason_Code;
+typedef int _Unwind_Action;
+
+struct _Unwind_Context;
+struct _Unwind_Exception;
+
 #ifdef __GNUC__
 #define LOG_UPCALL_ENTRY(task)                            \
     LOG(task, upcall,                                     \