When reserving stack space by decrementing rsp/esp, we were ignoring
the constant size needed for the stack marker and the wasm context.
Later, we were using that space anyway, which can lead to errors if e.g.
interrupt handlers kick in and use that space below rsp/esp.

R=ahaas@chromium.org

Bug: v8:7366, v8:6600
Change-Id: I2f49ef5785d33e98c29c5cf4fe7624a02e8c7628
Reviewed-on: https://chromium-review.googlesource.com/883881Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50843}