[libc++] SSH: Create a tarball of dependencies and scp that instead

[libc++] SSH: Create a tarball of dependencies and scp that instead

The benefit of doing this is that we can now handle directories that
contain symlinks and other arbitrary things, such as the static_test_env
required by filesystem tests.

As a fly-by fix, we also accumulate several commands to perform over SSH
and execute them at once instead of SSHing several times. This should be
faster on average.