PLIC SAIL

[ptprasanna]

Implementing PLIC on Sail model.

[billmcspadden-riscv]

I've given an initial review of Saravana's PR for the PLIC. There is a bit of work to be done on the Sail code, but not a lot.
There are some coding style issues that I flagged. As for functionality, we needs to add a command line switch to enable/disable the PLIC.

However, we need to have a set of tests written that test basic functionality. And they need to be added to the repo.