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

[Louis Dionne via cfe-commits]

ldionne wrote:

@ilovepi Some things that *could* be going wrong: The `cxx_shared` target is now defined when it wasn't before, and something in your CMake has e.g. `if (TARGET cxx_shared)` and does something it shouldn't based on that.

Otherwise, it's also possible that `libc++.so` doesn't get installed at the right location anymore in case it's not part of the `all` target under that configuration, but that seems unlikely because previously the target would not have existed at all.

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