Support the latest newlib, libc++ and GCC (replaces #442)

[ppannuto]

#442 has an unfortunate chicken-and-egg problem wherein a commit new enough to checkout by hash in the dockerfile will only exist in the main libtock-c repo if the pull request comes from a branch within the libtock-c repo.

Hence this PR, which is based on the external branch, but is now coming from an internal branch. This isn't the most ergonomic or friendly solution, but this is also a rare enough issue [hopefully] that a little occasional human labor seems the path of least resistance.

I added commits to update the repo path, remove the unneeded patch file, and update the hash to check out.

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~~TODO: Technically, the Makefile change to only patch if there's a patchfile is untested as newlib is still building on my laptop, but I doubt there will be any issue~~ Hubris will get you every time... https://github.com/tock/libtock-c/pull/470/commits/ceda51b15c125599ee90bb61ce61ee97e0103cfa