document differences in kexec and Linux boot flows between architectures

[orangecms]

First code is often: arch/$ARCH/kernel/head.S

x86

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/machine_kexec_64.c?h=v6.15-rc6#n53
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/x86_init.c?h=v6.15-rc6#n123

arm

has no kexec_file_load yet, WIP was there

https://lore.kernel.org/linux-arm-kernel/20200601204127.GS1551@shell.armlinux.org.uk/

arm64

tbd

riscv

tbd