[lldb/gdb-remote] Change default value of use-libraries-svr4 to true

This setting was added last year, defaulting to false. There have been
no bug reports about the svr4 code path since then, and the using this
packet is definitely faster than walking the module list from lldb.

Set the default value of the setting to true, as that is a better
default. Users can still change it back if encountering problems, or we
can revert the change as well, in case of bigger issues.

I also add a note to the setting description that it is only effective
if lldb is built with xml support.