raspberrypi-kernel-headers 32bit lacks ARMv8 support

Reproduce the report on 32-bit Raspberry Pi OS by installing raspberrypi-kernel-headers and inspecting /lib/modules/$(uname -r), comparing the 6.1.21-v8+ and v7 directories. Trace how the package supplies kernel headers for each architecture; done means the 32-bit v8 directory contains the expected build headers as well as the v7 directories.

Describe the bug

On a Raspberry Pi 4 running 32-bit Raspberry Pi OS, installing the raspberrypi-kernel-headers package does not supply the required headers for the ARMv8 architecture. The /lib/modules/6.1.21-v8+ does not contain a "build" folder, while the v7 ones do:

pi@openwb:~ $ ls /lib/modules/*
/lib/modules/6.1.21+:
build              modules.builtin            modules.dep      modules.softdep
kernel             modules.builtin.alias.bin  modules.dep.bin  modules.symbols
modules.alias      modules.builtin.bin        modules.devname  modules.symbols.bin
modules.alias.bin  modules.builtin.modinfo    modules.order

/lib/modules/6.1.21-v7+:
build              modules.builtin            modules.dep      modules.softdep
kernel             modules.builtin.alias.bin  modules.dep.bin  modules.symbols
modules.alias      modules.builtin.bin        modules.devname  modules.symbols.bin
modules.alias.bin  modules.builtin.modinfo    modules.order

/lib/modules/6.1.21-v7l+:
build              modules.builtin            modules.dep      modules.softdep
kernel             modules.builtin.alias.bin  modules.dep.bin  modules.symbols
modules.alias      modules.builtin.bin        modules.devname  modules.symbols.bin
modules.alias.bin  modules.builtin.modinfo    modules.order

/lib/modules/6.1.21-v8+:
kernel             modules.builtin.alias.bin  modules.dep.bin  modules.symbols
modules.alias      modules.builtin.bin        modules.devname  modules.symbols.bin
modules.alias.bin  modules.builtin.modinfo    modules.order
modules.builtin    modules.dep                modules.softdep

When running 64-bit Raspberry Pi OS, the raspberrypi-kernel-headers contains the ARMv8 headers:

pi@raspberrypi:~ $ ls /lib/modules/6.1.21-v8+/
kernel             modules.builtin.alias.bin  modules.dep.bin  modules.symbols
modules.alias      modules.builtin.bin        modules.devname  modules.symbols.bin
modules.alias.bin  modules.builtin.modinfo    modules.order
modules.builtin    modules.dep                modules.softdep

Steps to reproduce the behaviour

1. Install Raspberry Pi OS bullseye 32-bit on a Raspberry Pi 4
2. Install raspberrypi-kernel-headers using apt
3. Investigate the /lib/modules/$(uname -r)` folder

Device (s)

Raspberry Pi 4 Mod. B

System

pi@raspberrypi:~ $ cat /etc/rpi-issue
Raspberry Pi reference 2023-05-03
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 7c750947a959fb626a70c09fd17c65815df192ac, stage2
pi@raspberrypi:~ $ vcgencmd version
Mar 17 2023 10:50:39 
Copyright (c) 2012 Broadcom
version 82f3750a65fadae9a38077e3c2e217ad158c8d54 (clean) (release) (start)
pi@raspberrypi:~ $ uname -a
Linux raspberrypi 6.1.21-v8+ #1642 SMP PREEMPT Mon Apr  3 17:24:16 BST 2023 aarch64 GNU/Linux

Logs

None applicable.

Additional context

No response