abc: new option to pass extra ABC read_lib args

[tefantasy]

This pull request adds an option -liberty_args to the abc command when performing technology mapping with liberty libraries. The option allows user to pass custom arguments to ABC's read_lib command.

This pull request can be regarded as a revival of #4343 but with minimal changes focusing on the -liberty_args feature and without other code restructurings (e.g., the introduction of user_script in #4343).

As noted in #4343, the main purpose of adding this option is to control how ABC generates its internal delay model from timing data in liberty files. This can improve mapping quality by enabling users to tune ABC’s delay modeling. For example, we tested on 12 IWLS'05/OpenCores designs, mapped to NanGate45 library. By adding -liberty_args "-G 250", the averaged (geometric mean) delay improvement (reported by post-synthesis STA) is 12.0%, at the cost of a 4.6% area increase.

Although #4343 has been replaced by #4592 which introduces larger scaled customization of ABC flow, it involves many new features and code refactoring, and possibly requires adding a new command. In contrast, this pull request makes a small modification to the existing abc command to increase the flexibility for mapping quality improvement.

[Goubermouche]

Seems sound to me, but please apply the suggested changes.