I'm using latest Phalcon stable (v.2.0.13) on a BCM2709 SoC. So it's ARMv7 architecture, thus 32bit only.
In a few days I'm getting new RPi 3 which has ARMv8 SoC, 64bits.
I expect Phalcon to compile on ARMv8 as well. Basically it's the compiler stuff - so your local GCC will transfer C sources into executable binaries for current machine.