AsyncReset: iverilog requires named initial block with `RANDOMIZE (#1807)

When simulating with iverilog, we get an error message for the
assignment to _RAND in the module's initial block:
"Variable declaration in unnamed block requires SystemVerilog."

Provide a name (":B0") for the initial block, which should be a
syntac no-op for already supported simulators, and allow also
running simulations under iverilog.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>