Add architecture-specific logic into extrapkg

This change will remove 4 needless sed lines from the PKGBUILD we use on
Arch for building this package and do the logic locally where it probably
makes more sense anyway. $CARCH should always be present as we fail if we
cannot find a correct makepkg.conf file in /etc/.

Signed-off-by: Dan McGee <dan@archlinux.org>