Use mkinitcpio's microcode hook instead of external microcode images

mkinitcpio now supports packing the microcode initramfs together with the main initramfs into one file. We should use this feature to simplify boot loader configuration. For PXE, this reduces the number of files that need to be transferred.

After a new official ISO with the change is released, the following things will need updating:

• https://github.com/archlinux/archweb/blob/master/templates/releng/archlinux.ipxe
• https://wiki.archlinux.org/title/Preboot_Execution_Environment#Existing_PXE_server