This changes the way {SaveFPRegsMode} is stored in a {CallDescriptor}.
The value is now encoded in the flags field, similar to all other
boolean properties. It removes the last mutable field from the call
descriptor, making the struct fully immutable.

R=neis@chromium.org

Change-Id: I6bca118c7ce0c5bc77e7aa2fcc6541439bd27b3b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1868616
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64512}