Add data-layout fields to build targets.

Created by: NilSet

Problem: Breaking change in rust-lang/rust#32939 makes the current nightly refuse to build without a data-layout field in the target json.

Solution: Take the data-layout emitted by clang for the target-triple and add it to the target json.

TODOs: Validate that clang's defaults are what we want. Documentation on data-layout format

Fixes: #620 (closed)