Debug builds would compile but not run correctly after rustc changed the
number of codegen units for debug builds to > 1 in this issue:

https://github.com/rust-lang/rust/pull/44853

Which landed in nightly-2017-09-30.

This patch works around the issue by resetting the default to 1.

It's not clear to me if the root cause of this issue is in our
code or in rust/llvm.