Strip *.so* and not only executable files

Our current stripping strategy requires that shared libraries have the
executable permission. However, this is by far not something
recognized as a standard behavior: Debian/Ubuntu distributions for
example do not have executable permissions on their
libraries. Therefore, pushing to upstream packages fixes that add the
executable permissions is not easy.

As a result, this commit improves the stripping logic so that it not
only strips the files that are executable, but also the ones that
match '*.so*', which should match both the shared libraries and the
dlopen()'able plugins, as long as they have a .so extension.

Thanks to this addition, a number of manual "chmod +x" done by various
packages can be removed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>