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

[Paul Kirth via cfe-commits]

ilovepi wrote:

> Also, this is another reminder that we would benefit from having Fuchsia builders in the pre-commit CI. We've asked before and we'll keep asking until we get them: Fuchsia is amongst the last platform that frequently reports errors but doesn't provide us with any pre-commit visibility.

Agreed. I'd also like us to be in that situation. 

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