This commit adds six new travis matrix entires for doing cross-compiled
distribution builds of the compiler. The support added in #38731 allows us to
quickly compile a complete suite of distribution artifacts for cross-compiled
platforms, and currently each matrix entry (when fully cached) clocks in around
an hour to finish. Note that a full test run typically takes about two hours
right now.

With further optimizations coming down the pike in #39026 this commit also
starts doubling up cross-compiled distribution builders on each matrix entry. We
initially planned to do one build per entry, but it's looking like we may be
able to get by with more than one in each entry. Depending on how long these
builds take we may even be able to up it to three, but we'll start with two
first.

This commit then completes the suite of cross-compiled compilers that we're
going to compile, adding it for a whole litany of platforms detailed in the
changes to the docker files here. The existing `cross` image is also trimmed
down quite a bit to avoid duplicate work, and we'll eventually provision it for
far more cross compilation as well.

Note that the gcc toolchains installed to compile most of these compilers are
inappropriate for actualy distribution. The glibc they pull in is much newer
than we'd like, so before we turn nightlies off we'll need to tweak these docker
files to custom build toolchains like the current `linux-cross` docker image
does.