[runtimes] Fix the build of merged ABI/unwinder libraries

Also, add a CI job that tests this configuration. The exact configuration
is that we build a shared libc++ and merge objects for the ABI library
and the unwinder library into it.

Differential Revision: https://reviews.llvm.org/D125903

GitOrigin-RevId: fa7ce8e685faa63f7ad1d0c7a640217286ec291b
3 files changed