Improve emulator.cc so that it will eventually tolerate async reset

As the comment in the source code explains in more detail, the
initial block in AsyncResetReg is either racy or is not handled
correctly by Verilator when the reset signal isn't a top-level pin.
I'm working around this by making sure that AsyncResetRegs will
see a rising edge of the reset signal, triggering the always block
instead of relying on the initial block.