VM tests only support one core, so have the other cores hammer on the
memory system to attempt to catch simple coherence regressions.

The supervisor code now runs in supervisor mode with negative virtual
addresses.  This further stresses VM and tests some RV64 corner cases.

This reverts commit 2397975c.

Refactoring support for custom instructions.

Enable support for the four custom instructions

* Update generated encoding.h (generated from riscv-opcodes)