[libcxx][lit] Add support for custom ssh/scp flags in ssh.py

In our CHERI Jenkins CI we need to pass `-F <custom_config_file>` to each
ssh/scp command to set various arguments such as the localhost port, usage
of controlmaster, etc. to speed up connections to our emulated QEMU systems.

For our specific use-case I could have also added a single --ssh-config-file
argument that can be used for both the scp and ssh commands, but being able
to pass arbitrary extra flags for both commands seems more flexible.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D84097