Support for CFLAGS_FOR_TARGET has disappeared in the GNAT run-time libraries but I don't know when. For sure, it is gone in the 4.4.x series. This is used by the recommended RTEMS GNAT build procedure.
Does that imply that Ada cross-compilers are broken?
If you need CFLAGS_FOR_TARGET set to find the files or specify target specific compiler options required to compile the run-time, then yes. This certainly breaks any cross build that is non-multilib and specifies target specific CPU model CFLAGS.
I built 4.6.x based GNAT cross toolsets so I am closing this.