5.4 Linux kernel on x86 panics with alignment exception when kexec'ed from 6.1 kernel

The boot chain here is UEFI -> 6.1.14 kernel built with EFI stub, initramfs loads and checks 5.4.109 kernel, then kexecs it. CPU is a 2-core Intel Atom N6210. When booted this way, at startup when the 5.4 kernel releases the second CPU, it immediately panics with an alignment exception. The same kernel image works fine when booted from EFI, or booted via kexec by the 5.4.109 kernel.