malloc: Allow falling back to legacy magic number on platforms which require it

[nc]

Summary

malloc: Fix kernel panic introduced by r367432 on certain platforms (e.g. AMD Ryzen) which erroneously report the legacy magic number as the version.

Submitted by: Neel Chauhan <neel AT neelc DOT org>
Tested on: HP Omen Obelisk 875-0142

[nc]

Panic screenshot (which this bug fixes): https://imgur.com/a/3dVHlwy

[nc]

This might have been an issue with the VirtualBox kernel module. Closing.