Having it around is an invitation to use it in new places. This CL
removes the generic Dummy descriptor and replaces it by other existing
descriptors if possible, and defines specialized dummies otherwise.

In the future, every builtin should have a real descriptor. Especially
new ASM builtins should define descriptors and use them in their
implementation (use Descriptor::FooRegister() instead of documenting
the calling convention as comments).

Change-Id: Ib577aa03b5e5a522460d1084cc9605c55cd29d6c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3585945
Auto-Submit: Jakob Linke <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80030}