gdb: factor out printing a table of solibs for info sharedlibrary

The next patch will add a new command that will print libraries in a
manner very similar to the existing "info sharedlibrary" command. To
make that patch simpler to review, this commit does the bulk of
refactoring work, since it ends up being a non-trivial diff to review.

No functional changes are expected after this commit.

Approved-by: Kevin Buettner <kevinb@redhat.com>