Removes the --ignition-staging flag since it is no longer used
by anything and won't be a shipping configuration. Also removes
ignition_turbo variant from testrunner, since it is now 
the same as the turbofan variant.

BUG=v8:4280

Change-Id: I3b96e986879fc70b8e202fe9496334828acdd0ba
Reviewed-on: https://chromium-review.googlesource.com/452621
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43806}

We did not associate any position to the stack check in the wasm
function prologue, hence a check failed later when trying to map the
non-existent position to the asm.js source position.

With this CL, we add a mapping to the source position table, mapping
the stack check call to byte offset 0 (which is distinct from any valid
instruction position). Also, we add another entry to the asm.js source
position sidetable, mapping byte offset 0 to the start source position
of the function body.

R=titzer@chromium.org, ahaas@chromium.org
BUG=chromium:677685

Review-Url: https://codereview.chromium.org/2609363004
Cr-Commit-Position: refs/heads/master@{#42130}