kernel since 5.6 do not boot anymore on Apple PowerBook

[Giuseppe Sacco]

Hello,
I am testing linux on a quite old hardware, an Apple PowerBook G4
"Titanium IV". I used to run Debian on this machine, so I upgraded the
old operating system to the latest software and found that new kernels
do not boot.
I rebuilt natively (on the powerbook) a few kernels and found that
5.4.50 still works fine, while 5.6.19 and 5.7.7 stop after printing:

pmac32_cpufreq: registering PowerMac CPU frequency driver
pmac32_cpufreq: Low: 667 MHz, High: 867 Mhz, Boot: 667 MHz

no more output is printed. I waited 5 minutes and then powered off the
machine using the power button. Rebooting with old kernel works.

So, I am looking for help. How may I better investigate the problem?
How may I crossbuild the kernel on a faster machine (amd64) in order to
try git-bisect without waiting ages?

Thank you,
Giuseppe