package/swig: create a legacy symlink for swig3.0

The host-swig package installs the swig binary as 'swig' and adds a
swig<major> symlink (E.G.  swig4.0).  This causes issues for older software
which may not know about the 4.0 version of swig, E.G.  CMake 3.10.x
contains the following swig detection logic:

find_program(SWIG_EXECUTABLE NAMES swig3.0 swig2.0 swig)

If the host has a 3.x or 2.x variant of swig installed, then that will be
used instead of our host-swig.

As a workaround, also add a swig3.0 symlink so our host-swig will be used.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
[Peter: reworded]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>