Moves AsmJs compilation into an AsmJs compilation job. This enables it
to be treated like other unoptimized compilations and avoids some
special-casing in compiler.cc.

BUG=v8:5203

Change-Id: I71ad27e3f72815b4c4074634fff0d168a9c89102
Reviewed-on: https://chromium-review.googlesource.com/581487Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46863}