We recently extended function-entry stack checks by an offset
representing the difference in optimized and unoptimized frame sizes,
with the intent of avoiding stack overflows during deopts. Although
the generated code is very efficient (just a single additional
register subtraction, executed exactly once per call), perf impact
is measurable.

To avoid the overhead in most cases, this CL adds a stack slack,
currently set to 256 bytes, by which deopts are allowed to exceed the
real V8 stack limit. For function-entry stack checks with an offset
less than stack slack, the offset is not applied and the more
efficient version of the stack check is emitted.

The V8 limit is chosen to be smaller than OS stack size (assumed to
be at least 1 MB). This guarantee is upheld even with slack.

Bug: chromium:1020989,v8:9534
Change-Id: Idee2e7ad1fa7810bf086a9f72ce00a9717010310
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1910099Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65025}