xtos: move sof/lib/perf_cnt.h to application interface

The SOF perf_cnt.h provides a simple performance counter interface that
is used in SOF to track performance at audio module and pipeline level.

Majority of the implementation is RTOS agnostic, relying on
sof_cycle_get_64() to sample platform clock, and timer_get_system() for
CPU clock, both defined in rtos/timer.h. There is however some
conditional rules for Zephyr to use timing_counter_get() if SOF is built
with CONFIG_TIMING_FUNCTIONS=y.

The amount of RTOS variation does not seem to warrant branching the
whole perf_cnt.h to RTOS layer. Move perf_cnt.h back to application
interface, so the single implementation can be shared.

Link: #9015
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>