[PATCH] One more step towards toplevel bootstrap

[Paolo Bonzini]

This patch adds support for toplevel bootstrap in the recursive targets. 
  That is, you can actually "make install" in a toplevel-bootstrap tree 
after this patch.

Also, this way "make bootstrap" can blindly invoke "make all" without 
playing strange games with stages.

What this does is to distinguish the *current* setup of the tree (saved 
in stage_current) and the setup of the tree that is to be used by 
recursive targets (saved in stage_last).  After bootstrapping is 
completed, the "gcc" directory is "stage3-gcc" but this is kept in 
stage_last: then other recursive targets can invoke recursively the 
stage3-start target and get back stage3 as "gcc".

This causes no change whatsoever in the Makefile for 
--disable-bootstrap.  Ok for gcc and src?

Paolo