That's probably the wrong TC for the job.
Never built uboot myself, so I can't vouch for it, but you probably need to use Amazon's TC, or build one that sticks as close to it as possible.
What you're using is much older than the already old TC you probably should be using

.
(A close fit is probably the final GCC 4.9 Linaro release).
Possibly critical sidebar: I'm not even sure if we actually can build uboot with a Linux TC, it might need a baremetal one?