Add INIT_ATTR to system_init()
On single-core targets, system_init() is only called from init()
in main.c, which is also INIT_ATTR, so this is safe.
Multi-core targets additionally call system_init() from cop_main(),
which isn't marked INIT_ATTR. I'm fairly certain that it would be
safe to add INIT_ATTR to cop_main() with a bit of refactoring, but
I don't have a way of testing it. So for now, leave INIT_ATTR off
for multi-core targets.
Change-Id: I8fe4358f975880ca317d46bda01d7d9db529d3e3