For WASM import calls to JSFunctions where the arity is mismatched,
we currently generate code that inlines the formal parameter count
of the target function as a constant in a call to the arguments
adapter. This CL changes this to generate code that loads the formal
parameter count from the function at runtime in order to permit
more sharing later.

R=mstarzinger@chromium.org
CC=clemensh@chromium.org

Change-Id: I5cce97fc338f6468f9d42d48f5bc860b25fb7d73
Reviewed-on: https://chromium-review.googlesource.com/1243108
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56220}