[libc++] [test] Generate static_test_env on the fly

Summary:
Instead of storing `static_test_env` (with all the symlinks) in the repo, we create it on the fly to be cross-toolchain-friendly. The primary use case for this are Windows-hosted cross-toolchains. Windows doesn't really have a concept of symlinks. So, when the monorepo is cloned, those symlinks turn to ordinary text files. Previously, if we cross-compiled libc++ for some symlink-friendly system (e. g. Linux) and ran tests on the target system, some tests would fail. This patch makes them pass.

Reviewers: ldionne, #libc

Reviewed By: ldionne, #libc

Subscribers: EricWF, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D78200