gdb/solib: use solib::ops for operations that concern a single solib

gdb/solib: use solib::ops for operations that concern a single solib

For operations that concern a single solib, use the solib_ops backlink
added in the previous patch (solib::ops), instead of using the solib_ops
from the gdbarch.  This is a small / easy step towards not using
gdbarch_so_ops, which is necessary for the C++ification patch later in
this series.

There is no change in behavior expected.

Change-Id: If80e9ea717a2788bada1cf0940cda3c73933bcff
Approved-By: Pedro Alves <pedro@palves.net>