Make sure generated functions returning DecimalVals adhere to x86 ABI

I hit this in the expr refactoring. This makes sure we never expose a
function that returns a DecimalVal directly (rather than through an
extra return parameter as specified by the ABI), which will crash if
called from precompiled native code.

Change-Id: Ifb249086c221b53553d3e7fb39af065f4cca2bac
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3425
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins