mkaurball: enforce sane permissions on tarballed files

There's not really any reason to set permissions to anything other than
644 or 755 (and even 755 is questionable). In the future, files will be
stored in git (where only the -x bits are stored), and this will need
to be dealt with (properly) in the PKGBUILD.

Fixes downstream bug: https://bugs.archlinux.org/task/41313