This is from the SysV calling convention not x86 itself. The CPU can do unaligned just fine. You don't have to use the calling convention when not calling out to a library.
You're right that it's not inherent to the architecture, but even if you're only calling your own code, if your own code is written in C, then GCC will assume it too, unless you use command-line arguments or attributes to tell it otherwise, neither of which is being done here.
