Previouly, we had 'name', 'symlink_name', and 'download_name'.
 - We are changing the S3 URL scheme to make 'download_name' redundant.
 - 'symlink_name' referred to the full target triple, and has been
   replaced with just 'triple'.
 - 'name' referred to an ambiguously shortened triple (which omitted
   the vendor field of 'unknown'), which is used by Ubuntu, and has
   been replaced with 'ubuntu_triple'.

Most of the references have been switched to use just 'triple', which
is the most specific identifier, with 'ubuntu_triple' used where
necessary (references to the /usr/bin/* cross-compilation binaries,
and references to the lib, usr/lib, and usr/include dirs extracted
from the ARM rootfs tarballs).
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