Simplify switch statement in ARM subtarget align access

This switch can be reduced to a simpler if/else statement.

Patch by Charlie Turner.
Refactor isThumb1Only() && isMClass() into a predicate called isV6M()

This must be enforced for all v6M cores, not just the cortex-m0,
irregardless of the user-specified alignment.

Patch by Charlie Turner.
Emit unaligned access build attribute for ARM

Patch by Charlie Turner.