Add "maint info linux-lwps" command

This adds a maintenance command that lets you list all the LWPs under
control of the linux-nat target.

For example:

 (gdb) maint info linux-lwps
 LWP Ptid        Thread ID
 560948.561047.0 None
 560948.560948.0 1.1

This shows that "560948.561047.0" LWP doesn't map to any thread_info
object, which is bogus.  We'll be using this in a testcase in a
following patch.

Co-Authored-By: Pedro Alves <pedro@palves.net>
Change-Id: Ic4e9e123385976e5cd054391990124b7a20fb3f5