This will make it easier to generate builtin calls that require the
context to be passed in that register, because this can be represented
as a {LiftoffRegister} then.

Drive-by: Fix a typo (x8 -> x28).

R=thibaudm@chromium.org

Bug: v8:11453
Change-Id: I7dcf7a0390dbde9713334d3da28ce6d79413a9f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2748585Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73324}