When calling imported functions, we were always using the global object
as receiver. This is incorrect for strict functions, which should have
undefined as receiver.
This CL fixes this also for the interpreter, making us pass
test/mjsunit/wasm/receiver.js with --wasm-interpret-all.

R=ahaas@chromium.org
BUG=v8:5822
TEST=test/mjsunit/wasm/receiver

Change-Id: Ib7d637083245f67b668c11540e3c3473bc167129
Reviewed-on: https://chromium-review.googlesource.com/465986
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44346}