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.

make memsize configurable

The registers can still hold invalid values, but an attempt to switch
to the mode stored therein will instead go to an implemented mode.

This is an unavoidably invasive commit, because it affects the unit tests
(which formerly exited using stop()), the test harness Verilog generator
(since it is no longer necessary), and the DRAM model (since it is no
longer connected).  However, this should substantially reduce the effort
of building test harnesses in the future, since manual or semi-automatic
Verilog writing should no longer be necessary.  Furthermore, there is now
very little duplication of effort between the Verilator and VCS test
harnesses.

This commit removes support for DRAMsim, which is a bit of an unfortunate
consequence.  The main blocker is the lack of Verilog parameterization for
BlackBox.  It would be straightforward to revive DRAMsim once support for
that feature is added to Chisel and FIRRTL.  But that might not even be
necessary, as we move towards synthesizable DRAM models and FAME-1
transformations.

We were relying on ALU passthrough for this, but failed to override the
ALU dw argument, so bits above 31 could be discarded.