gdb/solib-svr4: update solib when an inferior is not being executed (e.g. core files)

Loading a core file currently generates a warning when enabling `set
verbose on`:

    $ ./gdb -nx -q --data-directory=data-directory a.out -ex "set verbose on" -ex "core core.151894" -batch
    ...
    warning: platform-specific solib_create_inferior_hook did not load initial shared libraries.

The warning comes from infcmd.c:post_create_inferior() which initializes
the solib_ops.  The latter is in charge of calling solib_add() in its
implementation of solib_ops::create_inferior_hook.  The svr4_solib_ops
class, however, does not call solib_add (through enable_break) if the
inferior is not being executed.

This patch moves the call to solib_add() outside of the conditional
nested call.  No user-visible change should be expected, except for fact
that the warning will no longer be shown.

Change-Id: I5488dc166fdc985669422a0f1c0c2f43158cd8c4
Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>