No need for active messages in this context.

This works around HTIF limitations and can later be removed.

This is technically possible by running an RV32 supervisor on an
RV64 machine, but the M-mode and S-mode code would need to be
compiled and linked separately.

That procedure doesn't work correctly if hart 0 comes out of reset while
hart 1 is still in reset.

Fix --disable-fp-emulation.

Fix macros for emulating atomic operations (--disable-atomics).

Add missing #include for config.h

This has the side effect of testing the IPI mechanism.  Still not sure
this is the best approach to booting, but it works...