[AArch64] Add support for the .inst directive.

This has been implement using the MCTargetStreamer interface as is done in the
ARM, Mips and PPC backends.

Phabricator: http://reviews.llvm.org/D5891
PR20964

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220422 91177308-0d34-0410-b5e6-96231b3b80d8