[ARM] Set default alignment to 64bits

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

Patch by: Diogo Sampaio(diogo.sampaio@arm.com)

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

Change-Id: I5a62b766491f15dd51e4cfe6625929db897f67e3
llvm-svn: 367119