Use the correct syntax for the aarch64 ext instructions

Since Xcode 5.1, the apple tools actually support using the
official, correct syntax for the ext instructions. This syntax
is already used in a number of places already - use it consistently,
and get rid of the compatibility hacks.
Merge pull request #1134 from mstorsjo/aarch64-ext

Use the correct syntax for the aarch64 ext instructions