Maglev is currently enabled only on x64; BUILD.gn knows about this
through v8_enable_maglev, but v8_check_header_includes doesn't. It
thus tries to compile maglev files on platforms that don't have maglev
support yet.

Add an explicit maglev exclude rule until we support other platforms.

Bug: v8:7700
Change-Id: Iac991741c7d630dc4ed9f9fbf6df7656853cc743
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3494536
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79305}