Handle the case of asm.js modules that return a single function
instead of a collection of them.

R=mtrofin@chromium.org
TEST=mjsunit/asm/asm-wasm
BUG=v8:4203
BUG=v8:5356

Review-Url: https://codereview.chromium.org/2348383003
Cr-Commit-Position: refs/heads/master@{#39515}