In preparation for kRootRegister support on ia32.

Instead of pushing the register args first thing within the builtin to
free up needed scratch registers, we just pass the last 3 arguments on
the stack.

Drive-by: Update documentation of helper function.

Bug: v8:6666
Change-Id: I4a194d6885ac9cdfb9f5e66d687522442fae39ba
Reviewed-on: https://chromium-review.googlesource.com/1199025
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55585}