It appears in the cctest context, both the old STUB, and the current 
WASM_FUNCTION "just work"; however, in the upcoming off-the-gc wasm 
world, we codegen call sites differently on x64 - far calls for 
js-to-wasm (this case), and near calls otherwise.

Bug: 
Change-Id: Iebf8acf164f07742fc367b7bbf266913dbc60c46
Reviewed-on: https://chromium-review.googlesource.com/735131Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48889}