Use CPU host name for code generation on the JIT codegen, and forcibly disable AVX.

I'm not sure this is a better idea than letting LLVM use the "default" CPU model (presumably some kind of generic machine model?), but intuitively it should be able to select specialized optimization rules.
Is there an `avx2` feature?  I saw it mentioned in the host detection code in LLVM, but only applies to Haswell CPUs.