[libcxx] [libcxxabi] [libunwind] [runtimes] Always define cxx_shared, cxx_static & other targets (PR #80007)

[Louis Dionne via cfe-commits]

ldionne wrote:

@petrhosek Where is the cache generating that build?

The fix for this is:

```
# On embedded platforms that don't support shared library targets, CMake implicitly changes shared
# library targets to be static library targets. This results in duplicate definitions of the static
# library targets even though we might not ever build the shared library target, which breaks the
# build. To work around this, we change the output name of the  shared library target so that it
# can't conflict with the static library target.
#
# This is tracked by https://gitlab.kitware.com/cmake/cmake/-/issues/25759.
set(LIBCXX_SHARED_OUTPUT_NAME "c++-shared" CACHE STRING "")
set(LIBCXXABI_SHARED_OUTPUT_NAME "c++abi-shared" CACHE STRING "")
set(LIBUNWIND_SHARED_OUTPUT_NAME "unwind-shared" CACHE STRING "")
```

https://github.com/llvm/llvm-project/pull/80007