qemu: Convert target_name() to TargetInfo API

qemu: Convert target_name() to TargetInfo API

Have target_name() be a target-agnostic method, dispatching
to a per-target TargetInfo singleton structure.
By default a stub singleton is used. No logical change
expected.

Inspired-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250424222112.36194-3-philmd@linaro.org>