If this flag is set, all code will be executed in the interpreter.
At some point in the future we might want to have a variant for testing
all code with this flag. Currently, I am using it for finding
unimplemented functionality in the interpreter and testing existing
functionality.
Mjsunit tests in follow-up CLs will use this flag too.

R=ahaas@chromium.org, titzer@chromium.org
BUG=v8:5822

Change-Id: I1393b4aedbb9f9070e011362ec95cb89f296dab4
Reviewed-on: https://chromium-review.googlesource.com/453700
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43773}