[libc++] Simplify how we define the linker script for libc++

Trying to be generic didn't work properly because we had to special-case
some interface libraries that we didn't want in the linker script. Instead,
only look at the ABI and the unwinding libraries explicitly.

This should solve the issue reported by @dim in [1].

[1]: https://discourse.llvm.org/t/15-0-0-rc1-has-been-tagged/64174/22

Differential Revision: https://reviews.llvm.org/D131037