🎨 Set sane default library path for verilator

Created by: zarubaf

This PR improves the verilator flow a bit: In particular it is checked whether the RISCV environment variable is set as we are relying on it in many places. Second it sets the linker symbols of verilator to use the non-default $RISCV/lib path for also looking for shared objects (in particular fesvr). This should eliminate potential sources of error.