Change v64 datalayout in SPU.

The SPU ABI does not mention v64, and all examples
in C suggest v128 are treated similarily to arrays, 
we use array alignment for v64 too. This makes the 
alignment of e.g. [2 x <2 x i32>] behave "intuitively"
and similar to as if the elements were e.g. i32s.

This also makes an "unaligned store" test to be 
aligned, with different (but functionally equivalent)
code generated.

llvm-svn: 117360