[libc++][libc++abi] Serialize the enable_assertions Lit parameter in the generated config

This means that re-running with llvm-lit in that configuration will
work as expected. This also enables assertions in libc++abi in the
Generic-assertions CI job, which was disabled previously.

Differential Revision: https://reviews.llvm.org/D122597

GitOrigin-RevId: 65b1b3b961cf0337a007e5646431abf2b7aff334