[ARM] Set default alignment to 64bits

Summary:
The maximum alignment used by ARM arch
is 64bits, not 128.

This could cause overaligned memory
access for 128 bit neon vector that
have unpredictable behaviour.

This fixes: https://bugs.llvm.org/show_bug.cgi?id=42668

Reviewers: ostannard, dmgreen, srhines, danalbert, pirama, peter.smith

Reviewed By: pirama, peter.smith

Subscribers: phosek, thegameg, thakis, llvm-commits, carwil, peter.smith, javed.absar, kristof.beyls, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D65000

llvm-svn: 368288