[libc++] Generate symlinks in static_test_env on the fly

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.

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