qemu-system-mipsel regression, Linux generated with Buildroot does not boot anymore
Description of problem:
Buildroot Toolchain Builders try to release a new version. See a message from Thomas Petazzoni with the remaining issues:
https://lore.kernel.org/buildroot/20240730223542.273693e5@windsurf/T/#u

All toolchains generate a system that fails to boot:

Run /sbin/init as init process
process '/bin/busybox' started with executable stack
Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004

The interesting thing is that those images boot fine with Qemu v8.2.6,
but they fail to boot with Qemu v9.0.2.

I tracked it down to this commit:
commit 4e999bf4197ae3dc58b7092260f98146920a7469
Author: Richard Henderson <richard.henderson@linaro.org>
Date:   Sun Jan 28 15:58:52 2024 +1000

    target/mips: Pass ptw_mmu_idx down from mips_cpu_tlb_fill
    
    Rather than adjust env->hflags so that the value computed
    by cpu_mmu_index() changes, compute the mmu_idx that we
    want directly and pass it down.
    
    Introduce symbolic constants for MMU_{KERNEL,ERL}_IDX.
    
    Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
    Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

Unfortunately just reverting this commit in 9.0.2 does not help, Qemu segfaults on Linux Kernel boot then.