Support unaligned load/store on more ARM targets

[Anton Korobeynikov]

> We could be conservative and only enable when we're sure it'll work (L on
> v6, D+L on v7), but GCC seems to default to unaligned for all v6 and v7
> (except v6M), this is why I recommend it on.
When this option (unaligned access) was initially added the discussion
ended with the default being conservative and correct. Subtarget can
surely redefine it, if necessary (like Darwin).

So, the proper solution here would be:

1. Allow unaligned access on Linux
2. Add flag to control the behavior
3. Left the current conservative behavior for everything else

> The default doesn't matter much if we have the flag.
It does, otherwise the defaults would be set using random number generator :)

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