FTR, commands used to change arch=() and then verify the changes:

$ sed -Ei "/^arch=/{s/ *?['\"]?i686['\"]? *?//;s/ *$//}" */*/PKGBUILD*

$ grep -r i686 * | grep arch= | grep -v repos

$ svn diff | grep -A 1 ^-[^-] | grep -v -- -- | cut -c2- |
  while IFS=$'\n' read -r l1; IFS=$'\n' read -r l2; do
    echo "$l1 -> $l2"
  done | sort -u

+ suggested improvements from bug reports