CV32E40P requires rv32imc_zicsr

This PR is motivated by #1506 (closed).

Here, we are trying to do two things:

1. Set rv32imc_zicsr are the default march for gcc.
2. Put in a simple (?) check in the Makefile to ensure you are using an up-to-date version of the compiler that supports _zicsr.

...this is at the limit of my Makefile scripting skills :frowning_face: Please do not merge yet.