I was able to build the complete firmware image for RT-AC68u, but also having a problem just using the cross-compiler tools to build a simple program.
mhorn@merlin:~/uhubctl$ which arm-brcm-linux-uclibcgnueabi-gcc
/opt/brcm-arm/bin/arm-brcm-linux-uclibcgnueabi-gcc
mhorn@merlin:~$ cat hello.c...