bump mill to 0.11

In this commit, I bumped mill to 0.11. And make chisel be able to cross
aginst two versions: 3.6.0 and 5.0.0, which ease the downstream
users(chipyard) to be able to stay and old version, but also pave the
road of future Chisel 5 and Chisel 6 support in rocketchip.