gdb: rename target_so_ops to solib_ops

gdb: rename target_so_ops to solib_ops

I don't like the name `target_so_ops`, because:

 - The name `target` is so overloaded, and in this case it's not even
   related to target_ops or anything else called "target".
 - We do have an implementation that actually fetches solibs from the
   target (solib_target_so_op in solib-target.c), so it's confusing for
   the "base class" to be called target_something as well.

Rename to solib_ops.

Change-Id: I46a983d44e81400470e22deb09aaf26ad8a3587f
Approved-By: Tom Tromey <tom@tromey.com>