Fix quoting in ibex_config.py
Fix quoting in ibex_config.py

This didn't quite work properly for Xcelium (giving strings with an
embedded space: '-define IBEX_CFG_RV32M=ibex_pkg::RV32MSingleCycle').
It turned out not to matter because we were evaluating one time too
often in the shell, but we're about to stop doing that.

AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,lowRISC/ibex,lowRISC/ibex