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

[Petr Hosek via cfe-commits]

petrhosek wrote:

I tested this locally in a clean build but I'm seeing the following build error:
```
CMake Error:
  Running

   '/usr/local/google/home/phosek/fuchsia/prebuilt/third_party/ninja/linux-x64/ninja' '-C' '/usr/local/google/home/phosek/llvm/llvm-project/build/fuchsia/runtimes/runtimes-armv6m-none-eabi-bins' '-t' 'recompact'

  failed with:

   ninja: error: build.ninja:52568: multiple rules generate /usr/local/google/home/phosek/llvm/llvm-project/build/fuchsia/lib/armv6m-unknown-none-eabi/libc++.a
```

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