Executable code which is in the native mode the processor expects, for a modern x86_64 system virtually all start up with a real mode bios. Your executable will have to do whatever it needs to turn on the processor mode it likes and jump to a code segment after the multiboot header.
Multiboot, as the specification says, is based around a 32-bit x86 compatible CPU, but other portions could be extended. You should likely presume the following: everything is sized the same as it is for the 32 bit target system of the specification. Larger addresses will probably sign-extended, but verify that.
That's just at a glance though, please read the documentation.