debug: tracing: Fix build with renaming of reserved functions.

We renamed _sys_trace_thread_switched_in to
z_sys_trace_thread_switched_in, however we already had a function
named z_sys_trace_thread_switched_in.  So rename z_sys_trace... to
z__sys_trace...

Fixes: #15184

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>