This is necessary for the arguments adaptor, as there are only
5 gp registers available and a call to the arguments adaptor
trampoline that does not have the trampoline address as a immediate
needs 6 (4 arguments + esi as context + register to call through).

Bug: v8:6666
Change-Id: Ie96cf0352c323e07e0daf369953df8f4ee9acb81
Reviewed-on: https://chromium-review.googlesource.com/c/1283050
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56767}