[libc++abi] Improve error message when libunwind is missing from LLVM_ENABLE_RUNTIMES (#77991)

This doesn't actually prevent the build from failing, but it provides a
better diagnostic that explains what is needed in order to fix the build.

Fixes #77846
Fixes #77843

GitOrigin-RevId: 48e5b5ea92674ded69b998cf35724d9012c0f57d
1 file changed