tests/tcg: add concept of container_hosts

tests/tcg: add concept of container_hosts

While docker is nominally multiarch these days it doesn't mean our
distros actually package all cross compilers for all architectures.
The upcoming Debian bullseye release will improve things further. At
least for now we can get things like the 32 bit ARM compiler on it's
64 bit cousin.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210401102530.12030-4-alex.bennee@linaro.org>