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

rebuild with data-default-0.7.1.1, data-default-class-0.1.2.0, data-default-instances-base-0.1.0.1

rebuild with asn1-encoding-0.9.4, cgrep-6.6.15, data-default-0.7.1, data-default-class-0.1.1, js-jquery-3.0.0, microlens-0.4.4.3, pipes-http-1.0.3, scientific-0.3.4.7, shake-0.15.9, tagsoup-0.14

rebuild with ghc-8.0.1, base-prelude-1.0.1.1, cabal-install-1.24.0.0, cairo-0.13.2.0, cmark-0.5.2.1, data-default-0.7.0, data-default-class-0.1.0, extra-1.4.7, generic-trie-0.3.0.2, glib-0.13.3.0, gtk3 0.14.2 -> 0.14.3, haddock-api-2.17.2, haddock-library-1.4.1, js-jquery-1.12.4, memory-0.13, microlens-0.4.4.0, network-multicast-0.1.1, pango-0.13.2.0, simple-sendfile-0.2.24, stack-1.1.2, wai-3.2.1.1