The aim of this patch is to build and link an external unwinder for libc++abi. The approach is generalized as suggested in https://reviews.llvm.org/D99913#3173529.
I think the idea behind this patch is interesting and useful, however I'd like to model what we do for selecting the ABI library from libc++ here. We should have a HandleUnwindLibrary.cmake file that will create interface targets, and we can then link against those from libc++abi directly. Let's avoid introducing even more options like LIBCXXABI_EXTERNAL_UNWIND_SOURCE_DIR and LIBCXXABI_EXTERNAL_UNWIND_LIB_NAME -- instead let's put the logic for how to handle z/OS's unwind library in HandleUnwindLibrary.cmake directly.