Passing a pointer of the needed type, and then reading using
ReadUnalignedValue is pointless, since the compiler can assume
alignment of the pointer value.
This CL fixes the remaining external refs of wasm to take an Address to
a single buffer.

R=ahaas@chromium.org

Bug: v8:7570, v8:3770
Change-Id: If8a7324a4703e1e900cb3c5644baef207e6a371d
Reviewed-on: https://chromium-review.googlesource.com/1023406
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52754}