This matches x86_64's behaviour in common_trap_exit. (called from
thread_context_first_enter)
Currently thread_context_first_enter is only called when creating new
processes from scratch, in which case this doesn't change the actual
behaviour. But once thread_context_first_enter is called as part of
execve support, this will ensure the Thread's m_current_trap is set
correctly to the new trap frame.
