[libc++] Support -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on in CI

[libc++] Support -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on in CI

This fixes -isystem/-L/-Wl,-rpath paths when -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on
is used (https://reviews.llvm.org/D107799#2969650).

* `-isystem path/to/build/generic-cxx17/include/c++/v1`. `build/generic-cxx17/include/x86_64-unknown-linux-gnu/c++/v1 (__config_site)` is missing.
* `-L path/to/build/generic-cxx17/lib`. Should be `build/generic-cxx17/lib/x86_64-unknown-linux-gnu` instead

Reviewed By: ldionne, phosek, #libc

Differential Revision: https://reviews.llvm.org/D108836