[libcxx] [libcxxabi] [libunwind] [libc++abi][libunwind] Run c++abi and unwind tests against a fake install root (PR #110171)

[Björn Pettersson via cfe-commits]

bjope wrote:

Seeing same fault as @zeroomega .

I think problem is related to LLVM_ENABLE_PER_TARGET_RUNTIME_DIR. When that is set to ON the install path is extended (in libunwind/CMakeLIsts.txt):
```
  set(LIBUNWIND_TARGET_SUBDIR ${LLVM_DEFAULT_TARGET_TRIPLE})
  if(LIBUNWIND_LIBDIR_SUBDIR)
    string(APPEND LIBUNWIND_TARGET_SUBDIR /${LIBUNWIND_LIBDIR_SUBDIR})
  endif()
```
And then the substitution in libunwind/test/configs/cmake-bridge.cfg.in is wrong:
```
config.substitutions.append(('%{lib}', '@LIBUNWIND_TESTING_INSTALL_PREFIX@/lib'))
```

https://github.com/llvm/llvm-project/pull/110171