Indeed the AUR packages needed for the toolchain can't be build since a couple of weeks which is a pity. The problem evolved during a transition to glibc from eglibc which has been abandoned.
The predecessor packages using
eglibc still build, though. (eglibc.org providing the source code seemed to have vanished a couple of weeks ago but is online again.) I still have them available, so if you're interested let me know. (Should be possible to
fetch them from some mirror, too. Never tried this.)
But I wonder whether it still is necessary to compile U-Boot and kernel your self at all.
Meanwhile there's a package uboot-a20-olinuxino-micro which should be able to boot linux-sun7i on this device.
Also, you could try linux-armv7. This one needs a more recent U-Boot flavour, e. g.
custodian u-boot-sunxi which isn't available from the repos afaik. I built it using an eglibc based toolchain some weeks ago and it works well on a Cubietruck. Note that upstream U-Boot switched to Kconfig which involves some minor changes during build.