gallivm: Initialize LLVM Modules's DataLayout to an empty string.

gallivm: Initialize LLVM Modules's DataLayout to an empty string.

This fixes crashes in llvmpipe with LLVM 3.8 and also some piglit tests
on radeonsi that use the draw module.

This is just a temporary solution.  The correct solution will require
creating a TargetMachine during gallivm initialization and pulling the
DataLayout from there.  This will be a somewhat invasive change, and it
will need to be validatated on multiple LLVM versions.

https://llvm.org/bugs/show_bug.cgi?id=24172

Reviewed-by: Roland Scheidegger <sroland@vmware.com>