[PATCH] Re-enable atomic support on AArch64

[Tim Northover]

Hi Renato,

> This seems an odd change... Is it 2 the new "default" value?

The "2" means that the type is properly aligned and atomic operations
always lock-free (according to lib/Frontend/InitPreprocessor.cpp). "1"
is only partial support. In this case i386 has a 64-bit "long long"
but doesn't put requirements on its alignment. I don't think that
should be the default position.

Tim.