sim: serial2console crashes, if sys_clk has a different name

[Hans Baier]

If I use more than one clock in a design (multiple .add_clocker() calls),
then verilator won't compile anymore because there seems to be a naming conflict between
the sys_clk wire and the pad.
That could be resolved by renaming the sys clock pad to something like "sys_clkin".
But then the serial console crashes, because in its initialization,
the sys_clk name is hardwired, and the clock of the serial console then becomes a null pointer.