This CL removes the 'v8_enable_embedded_builtins' GN argument and sets
the 'V8_EMBEDDED_BUILTIN' define unconditionally.

Removing the now obsolete 'define' and corresponding C++ code will be
done in a separate CL.

R=machenbach@chromium.org

Bug: v8:8519
Change-Id: Ibaa3e958121931c0ce14baf48ad401fab87c5337
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1864929
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64456}