- Adds kFPReturnRegister0 for all platforms.
- Reworks linkage.cc to assign return registers to the proper
  register type, using the new FP return register.

Bug: v8:10070

Change-Id: I5c876d248de9b825a1c80847ab134881dcda6f04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2107510Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67183}