[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

NOKEYCHECK=True
GitOrigin-RevId: fa7ce8e685faa63f7ad1d0c7a640217286ec291b
1 file changed
tree: 01ac0f3ea95636f5e527e49e56fe97d6fbea73e5
  1. cmake/
  2. docs/
  3. include/
  4. src/
  5. test/
  6. .clang-format
  7. CMakeLists.txt
  8. LICENSE.TXT