Add `fpnew` as git submodule

Created by: sabhiram

Hey folks, thanks for all the work on this.

We currently do some systemc work that requires a riscv model which this repo provides nicely. In trying to automate some of our build-flow we ran across this somewhat icky git clone being done from a Makefile inside a source directory.

I took the liberty to clean this up somewhat in our fork - please review if you wish, and do with this PR as you see fit :)