Now that wasm-linkage.h is split off, we can easily implement
{MoveToReturnRegister} in platform independent code.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I072a0ee48d58ed29e0df489016f838915c3f2cb2
Reviewed-on: https://chromium-review.googlesource.com/1041690
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52939}