Control Spike parameters from cmdline.

Created by: zchamski

• vendor/riscv/riscv-isa-sim/riscv/Params.h (Param::setFromCmdline): New.
• vendor/riscv/riscv-isa-sim/spike_main/spike.cc (main): Declare 'params'
  variable ahead of cmdline options. Add cmdline option '--param PATH=VALUE'.
  Add corresponding help string.