linux-user: ensure nios2 processes queued work

linux-user: ensure nios2 processes queued work

While async processes are rare for linux-user we do use them from time
to time. The most obvious one is tb_flush when we run out of
translation space. We will also need this when we move plugin
vcpu_init to an async task.

Fix nios2 to follow its older, wiser and more stable siblings.

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: <20240227144335.1196131-19-alex.bennee@linaro.org>