Revert "arm64: Don't use uxth for extending a register"

This reverts commit 91e0b478.

It turns out that this wasn't a new bug specifically for uxth, but gas-preprocessor only had a workaround for uxtw, not for the shorter forms.

With https://lists.libav.org/pipermail/libav-devel/2018-October/086390.html in place, this workaround isn't necessary. Sorry for not noticing this before sending the MR for the workaround...