tests: benchmarks: timing_info: fix start of swap time

In all architectures, except x86, __start_swap_time is directly
updated by function read_timer_start_of_swap(), and it shall not
be registered via __temp_start_swap_time.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>