Add a data layout specification

[dzagieboylo]

From @gharrma on February 3, 2017 6:29

"Make sure that your Modules contain both a data layout specification and target triple. Without these pieces, non of the target specific optimization will be enabled. This can have a major effect on the generated code quality." -- LLVM Performance Tips

Copied from original issue: gharrma/polyllvm#16

[dzagieboylo]

From @gharrma on February 23, 2017 6:25

Unfortunately it seems that there's no automatic way to get the LLVM target triple and data layout of the host machine. Even the front end for Rust appears to specify data layouts manually for all supported targets.

Our best option may be to do something similar (for the most common platforms), then allow users to pass in their own target triple and data layout.