ARM is interesting because of the customisability of ARM chips. Some companies, such as Apple, license the ISA and spin their own silicon. Others license the whole CPU design from ARM holdings, and ARM produce several lines of designs with different CPU capabilities.
Edit: Looks like ARM64 was affected, but it has an architectural feature that makes the mitigation much easier: http://lists.infradead.org/pipermail/linux-arm-kernel/2017-N...