gitlab: add build-loongarch to matrix

We have the compiler and with a few updates a container that can build
QEMU so we should at least run the check-tcg smoke tests.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231029145033.592566-6-alex.bennee@linaro.org>
4 files changed