tests/tcg: enable basic testing for aarch64_be-linux-user

tests/tcg: enable basic testing for aarch64_be-linux-user

We didn't notice breakage of aarch64_be because we don't have any TCG
tests for it. However while the existing aarch64 compiler can target
big-endian builds no one packages a BE libc. Instead we bang some
rocks together to do the most basic of hello world with a nostdlib
syscall test.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241023113406.1284676-14-alex.bennee@linaro.org>