[nrf fromlist] samples: boards: nordic: coresight_stm: Benchmark only the cached code

Add additional call before starting time measurement to ensure that
all calls that are benchmarked are already cached. This change allows
to get more stable results as all exectutions are from cache.

Upstream PR #: 100784

Signed-off-by: Krzysztof Chruściński <[email protected]>

Author: nordic-krch

samples/boards/nordic/coresight_stm/src/main.c

@@ -16,6 +16,7 @@ LOG_MODULE_REGISTER(app);
 #define TEST_LOG(rpt, item)                                                                        \
 	({                                                                                         \
 		uint32_t key = irq_lock();                                                         \
+		__DEBRACKET item;                                                                  \
 		uint32_t t = k_cycle_get_32();                                                     \
 		for (uint32_t i = 0; i < rpt; i++) {                                               \
 			__DEBRACKET item;                                                          \