Disable vectorization on pre SSE2

[Jasper Neumann]

Hello Elena, hello all!

 > Does anyone today have a pre SSE2 computer?
 > SSE2 was introduced more than 10 years ago.

My Raspi does not have SSE2, and I have a lot of ancient x86 computers 
on my attic with processors such as 8088, 80286, 80386. :-)

Earnestly: Please keep in mind that there are brand new x86 CPUs such as 
Intel Quark which do not have any vector unit but ought to be a valid 
target for LLVM.

Jasper