[libunwind] [libcxx] [libcxxabi] [runtimes] Don't link against compiler-rt explicitly when we use -nostdlib++ (PR #75089)

[Petr Hosek via cfe-commits]

petrhosek wrote:

> > I'm trying to implement support for building libunwind, libc++abi and libc++ against LLVM libc in which case we won't be able to rely on `-nostdlib++`, we'll need to use `-nostdlib` to avoid linking the C library. We can still use `-nostdlib++` when LLVM libc isn't being used used, but a lot of this logic will need to be refactored to support the new use case. With that in mind, I'm fine with change as an interim solution.
> 
> Is there a reason why `-nostdlib` also drops compiler-rt? If `-nostdlib++` affects only the C++ library, it would make sense that `-nostdlib` affects only the C library?

The reason is matching the behavior of GCC, but given that I don't think we can change the current semantics.

https://github.com/llvm/llvm-project/pull/75089