Writing Portable ARM64 Assembly

An unfortunate side effect of the rising popularity of Apple's ARM-based computers is an increase in unportable assembly code which targets the 64-bit ARM ISA. This is because developers are writing these bits of assembly code to speed up their programs when run on Apple's ARM-based computers, without considering the other 64-bit ARM devices out there, such as SBCs and servers running Linux or BSD.
The good news is that it is very easy to write assembly which targets Apple's computers as well as the other 64-bit ARM devices running operating systems other than Darwin.