arachne-pnr switched to nextpnr for basic Verilog examples

[camrbuss]

Updated Makefile's in basic Verilog examples to use nextpnr instead of arachne-pnr. Examples that used more advanced make configurations were left untouched as well as examples that had arachne or nextpnr options.

The following examples were updated, tested for build, and programmed onto the ICESugar V1.5 board:

• dsp
• hard_rgb
• leds
• pmod_leds
• pmod_switch
• pwm
• spram
• switch
• uart_tx