plugins/loader: compile loader only once

plugins/loader: compile loader only once

There is very little in loader that is different between builds save
for a tiny user/system mode difference in the plugin_info structure.
Create two new files, user and system to hold mode specific helpers
and move loader into common_ss.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250304222439.2035603-26-alex.bennee@linaro.org>