More robust implementation for finding the beginning of the stack.

This patch inserts an extra stack frame above the function that
calls the actual Nimrod code and ensures that a reference to this
frame is stored as the stack bottom.

lib/system.nim

@@ -2115,6 +2115,12 @@ when not defined(JS): #and not defined(NimrodVM):
         locals = addr(locals)
         setStackBottom(locals)
 
+    proc initStackBottomWith(locals: pointer) {.inline, compilerproc.} =
+      # We need to keep initStackBottom around for now to avoid
+      # bootstrapping problems.
+      when defined(setStackBottom):
+        setStackBottom(locals)
+
     var
       strDesc: TNimType