Improve Verilator generated by make model-lib

Created by: jeremybennett

The library version of the Verilator model is likely to be included
in shared object libraries, so is compiled with -fPIC. The typical
applications will be focussed on performance rather than debugging
the Verilator model, so it is also compiled with -O3.

* README.md: Extend documentation for the model-lib target.
* core-v-mcu.core: Build verilator model for model-lib target with -O3 and -fPIC.

Signed-off-by: Jeremy Bennett jeremy.bennett@embecosm.com