How so? The bootloader already runs in long mode with UEFI and it also takes care of bringing up other cores so this is not really a problem in my experience.
UEFI doesn't really take care of bringing up the other cores. There's a way to get access to them, but you have to give them back to UEFI before exiting boot services, so that doesn't really help your kernel. You still need to do the SIPI into x86-16 code to really take control of the AP cores.
Even the new way (that isn't actually implemented AFAIK) just SIPIs into long mode code, it doesn't use the UEFI multicore stuff.
Even the new way (that isn't actually implemented AFAIK) just SIPIs into long mode code, it doesn't use the UEFI multicore stuff.