Support unaligned load/store on more ARM targets

[Anton Korobeynikov]

> Let's be conservative in general and make Linux more performing.
I think we can actually make everything generic:

1. Define subtarget feature. Let's call it "allow unaligned access".
Make it false by default
2. Enable it for Darwin and Linux / PNaCl + v7
3. Check for feature, not for subtarget here

This way we can even enable/disable unaligned accesses on per-function
basis using new target function attributes.

What do you think?

--
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University