scripts: load CLKGATE_MAP_FILE as lib and techmap it

[widlarizer]

The main idea is that if you use yosys clockgate to map ICGs to OPENROAD_CLKGATE, you need a blackbox to be present, which is missing at that time since currently it's loaded with -defer. That leads to check failures due to missing cells after abc which this PR fixes. Credit for the idea goes to @povik