Reapply r359582 (by emaste):

lldb: use lua as the default script language

In the FreeBSD base system we do not have Python support in lldb, but
will have Lua support. Make Lua the default.

This needs to be made into a configure-time option; that is being
discussed upstream and will appear in a future lldb import. For now
carry this change as a tiny patch to our copy of lldb.