[VTA][TSIM] Verilator compile report error for printf (#3438)

[VTA][TSIM] Verilator compile report error for printf (#3438)

[Symptom]
after follow the tsim example readme, doing verilator install by 'sudo apt-get-install verilator'
Once enable 'debug' or manually add 'printf' logic in chisel module, verilator would report
following error.
'syntax error, unexpected INTEGER NUMBER, expecting IDENTIFIER'

[Fix]
upgrade verilator to 4.012, issue fixed.

[Solution]
Link README.md verilator install steps with verilator home website
install instruction.