THe problem is classic chicken-egg problem. The gnat bootstrap has to be built with native gnat compilier, the compiler must have current or previous version (gnat 4.8 can be built using gnat 4.7 and so on). However Fedora ARM doesn't contain gnat compilers at all.
We've build native compiler using cross-compilation and rebuild gcc using the compiler on the native platform. RPMs can be found here: [fedorapeople](http://landgraf.fedorapeople.org/gnat_arm/). To make it in koji gnat* packages should be installed before gnat is build. The compliler should successfully rebuild gcc itself with [patch] (http://gcc.gnu.org/viewcvs/gcc?view=revision&revision=205681) applied and packages from the Ada stack (xmlada and gprbuild at least).