Taking similar approach as ia32 which also has 1 return register
eax (as per ia32's ABI) but uses edx as return register as well.
This will fix some failures on s390x where a function returns 2
values.

R=titzer@chromium.org, bmeurer@chromium.org

BUG=
LOG=N

Review-Url: https://chromiumcodereview.appspot.com/2426233002
Cr-Commit-Position: refs/heads/master@{#40439}