Add reloc keyworkd to let EFI bootstrap load amd64 kernel at any address

EFI bootstrap assumes it can copy the amd64 kernel to its ELF load
address (that is KERNTEXTOFF - KERNBASE = 0x200000), but it can
clash with previous UEFI memory allocation, as described here:
http://mail-index.netbsd.org/tech-kern/2023/04/07/msg028833.html

This change adds a reloc keyword for controling where the EFI
boostrap will copy the kernel image. Possible values are:
default - the default and prior behavior, copy at 0x200000.
none - do not copy and use the kernel image where it was loaded.
address - specify an explicit address where to copy the kernel.

This comes with an amd64 kernel patch that makes it self-relocatable.
It first discover where it was loaded in memory, and if this is
different than the expected 0x200000, hhe the kernel relocates
itself and start over at the right address.

branches: 1.2.2; 1.2.6; 1.2.14; 1.2.18;
efiboot: Don't access old stack after copying a kernel.

It's possible that the old stack is overwritten by the kernel.

fix to be able to boot amd64 kernel from 32bit efiboot (booia32.efi).

efiboot: Don't access old stack after copying a kernel.

It's possible that the old stack is overwritten by the kernel.

fix to be able to boot amd64 kernel from 32bit efiboot (booia32.efi).