Fix support_verilator_4 detection for bash and add optimization to verilator

Created by: xushengj

On our system with bash, even if we have verilator 4 properly set up, ariane's Makefile will not set support_verilator_4, leaving the verilator output to be single threaded. This patch adds our fix to this issue. In addition, -O2 option is added to CFLAGS when compiling verilate target.