Simplify handling of Liberty file related variables

[widlarizer]

• remove preprocessLib.py
  • yosys-abc accepts user-defined attribute original_pin used in Liberty file pin declarations of cells it actually uses to map to even if original_pin is not defined as user-defined. Probably from Synopsys PrimeTime folklore? Can't find much of a trace. Let me know if private PDKs break with this
  • yosys accepts the Liberty expression pattern described in the comment, see here
  • extract gz in Makefile instead (could be done more elegantly but I don't like fiddling with Make)
• rename DONT_USE_LIBS->LIB_FILES_EXTRACTED
  • it's the files listing cells that should be used, but it was named the same as the list of cells that shouldn't be used. This is because before #1813 there was a script that would add dont_use attributes to some of the cells in those files
  • the only difference between LIB_FILES_EXTRACTED and LIB_FILES is that they're extracted if gzip and copied to objects/
• rename DONT_USE_SC_LIB->SC_LIB_FILE

I tried this out with asap7 and nangate and it seems to work fine

cc @povik