With this CL we emit safepoint maps for externref values on the Liftoff
value stack. With that there is support for externref parameters and
locals in Liftoff, as well as for intermediate values of type
externref.

R=thibaudm@chromium.org

Bug: v8:7581
Change-Id: I2df0a8d00b2da33fe06ff474b039cca4c7be726d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2387571
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69725}