For this, all Torque stub-linkage builtins use TFC instead of TFS,
with a custom descriptor added to interface-descriptors.h

To avoid having complex logic in the generated code, the new class
TorqueInterfaceDescriptor contains the logic to create a
CallInterfaceDescriptor from a signature consisting of TNode types.

As an example and test, this CL ports StringCharAt to Torque.

Bug: v8:7793
Change-Id: I8339d2ad6e4f908ebdc3b8d30244e4bcbd974f21
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1798427Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64148}