[ARM] ARMv6m and v8m.baseline strict align

This sets target feature FeatureStrictAlign for Armv6-m and Armv8-m.baseline,
because it has no support for unaligned accesses.
It looks like we always pass target feature "+strict-align" from
Clang, so this is not a user facing problem, but querying the subtarget
(in e.g. llc) for unaligned access support is incorrect.

Differential Revision: https://reviews.llvm.org/D48437

llvm-svn: 335326