[libunwind] Fix path to libunwind for per-target-runtime-dir builds

[libunwind] Fix path to libunwind for per-target-runtime-dir builds

We recently introduced a from-scratch config to run the libunwind tests.
However, that config was always looking for libunwind in <install>/lib,
and never in <install>/<target>/lib, which is necessary for tests to
work when the per-target-runtime-dir configuration is enabled.

This commit fixes that. I believe this is what caused the CI failures we
saw after 5a8ad80b6fa5 and caused it to be reverted.

Differential Revision: https://reviews.llvm.org/D112322