Not using Arch Linux, but just from your post it looks like it's not finding the libphobos.a from HEAD and using the system one instead.You may want to check the dmd.conf file for your HEAD D install: make sure DMD is using it (dmd | head -4), and that it adds the library search path correctly (-L-L switch in DFLAGS).
Thanks! Typo in dmd.conf... sigh. Atila