GitLab CI: Make wider use of tags

GitLab CI: Make wider use of tags

First, introduce DEFAULT_ALL_TAG, DEFAULT_ARM64_TAG, DEFAULT_AMD64_TAG
and DEFAULT_FAST_AMD64_TAG and remove the previous DEFAULT_TAG (as
anyone making use of that will need to adjust their jobs). This allows
us to say that some jobs can run on amd64 or arm64 hosts under the all
tag, while some jobs must run on amd64 (the Xtensa jobs due to
binary-only toolchains and sandbox for now) Then we rework the world
build stage to only run on our very fast amd64 hosts, or our arm64 hosts
(which are also very fast). This should result in a similar overall
build time but also a much more consistent overall build time as we
won't have the two big world jobs possibly run on our slower build
nodes.

Signed-off-by: Tom Rini <trini@konsulko.com>