Pass multiple lib files to ABC

[rovinski]

Yosys allows for passing though multiple .lib files to ABC since version 0.41. Therefore, external liberty merging is no longer required in any part of the flow. This PR removes liberty merging and updates variable names

• Change DONT_USE_* variables to PROCESSED_* variables to better reflect their meaning (we stopped modifying dont_use properties in lib files since #1813)
• Remove the mergeLib rule in Makefile since there are no more users of it
• Pass multiple libs into the abc options
• Add a default DFF_LIB_FILE for asap7 because it appears that it was always supposed to have one, but it instead relied on the FF files being merged into the standard cell files

If/when dfflibmap supports reading multiple lib files, we can completely remove DFF_LIB_FILE and PROCESSED_SC_LIB variables. Restructure may lose some support but the underlying abc version would just have to be updated.