[ARM] Add poly64_t on AArch32.

Summary:
The poly64 types are guarded with ifdefs for AArch64 only. This is wrong. This
was also incorrectly documented in the ACLE spec, but this has been rectified in
the latest release. See paragraph 13.1.2 "Vector data types":

https://developer.arm.com/docs/101028/latest

This patch was written by Alexandros Lamprineas.

Reviewers: ostannard, sdesmalen, fpetrogalli, labrinea, t.p.northover, LukeGeeson

Reviewed By: ostannard

Subscribers: pbarrio, LukeGeeson, kristof.beyls, danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79711