After dropping support for older GCC versions, we can now assume full
c++14 constexpr support, and can hence resolve a TODO to make a
LiftoffCompiler method constexpr.

This is also a proof-of-concept to verify that this indeed works on all
bots now.

R=ahaas@chromium.org

Bug: v8:9686, v8:11384
Change-Id: I9b7da91152b0af98778312d653aebbf4236fed0d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2876850Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74407}