... and CallParameters::arity().

The construct arity contains the actual argument count, plus 2 for the
target  (the first input) and new target (the last input). This CL adds
a named constant and a helper method for accessing arity without extra
args. In the future we may want to remove the extra args from arity()
altogether.

Call arity is similar but includes the target and receiver.

Bug: v8:10542,v8:8888
Change-Id: I850fa314f88c2bee9d4dcd87eac9295b2bf88281
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208850
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67963}