This gets some leaf-level parameters out of the global parameterization,
better separating concerns. This commit also allows disabling the
M extension.
Heads up @colinschmidt and @ccelio. I'm removing these because
they are ISA constants and so are not truly parameters, so the
parameter place is not the place for them. Since BOOM and Hwacha
both depend on rocket, you should be able to obtain them by
instantiating/extending rocket.HasCoreParameters.
Andrew Waterman
Howard Mao