[libc++] Allow specifying arbitrary custom executors with the new format

[libc++] Allow specifying arbitrary custom executors with the new format

The integration between CMake and executor selection in the new format
wasn't very flexible -- only the default executor and SSH executors were
supported.

This patch makes it possible to specify arbitrary executors with the new
format. With the new testing format, a custom executor is just a script
that gets called with a command-line to execute, and some arguments like
--env, --codesign_identity and --execdir. As such, the default executor
is just run.py.

Remote execution with the SSH executor can be achived by specifying
LIBCXX_EXECUTOR="<path-to-ssh.py> --host <host>". Similarly, arbitrary
scripts can be provided.