Fix 4-state value support for $readmem (#5070)

[sifferman]

Fixes https://github.com/verilator/verilator/issues/5070

(I still need to add a couple tests, but I'm posting the PR now in case you have early suggestions).

Changes:

• A z or x in a data string will be converted to a random bit/nibble according to $readmemb/$readmemh and +verilator+rand+reset
• A 4-state value in an address will now result in a syntax error.