fix(benchmarks): use nsi_host_clock_gettime to bypass all native_sim time interception

native_sim intercepts both clock_gettime() AND the syscall() C function to
return simulated Zephyr time. The raw syscall approach also failed because
native_sim overrides the syscall() C function itself.

nsi_host_clock_gettime() is native_sim's own built-in escape hatch: it calls
the real host OS clock_gettime() via the original libc function that was saved
before any intercept. This is the correct and documented way to get real host
time from within native_sim firmware code.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: tbitcs

tests/benchmarks/kalman_benchmark/src/main.c

@@ -19,17 +19,17 @@
  * Zephyr kernel ticks which do NOT advance during CPU-bound loops.
  */
 #ifdef CONFIG_NATIVE_SIMULATOR
-/* native_sim replaces the C-library's clock_gettime() with one that returns
- * simulated Zephyr time (stuck at 0 during CPU-bound loops). A raw Linux
- * syscall bypasses the intercept and reads the real CLOCK_MONOTONIC directly
- * from the kernel, giving true nanosecond-resolution wall-clock timing.
+/* native_sim replaces clock_gettime() AND syscall() with versions that
+ * return simulated Zephyr time (stuck at 0 during CPU-bound loops).
+ * nsi_host_clock_gettime() is native_sim's own escape hatch to the real
+ * host OS clock, bypassing all simulated-time interception.
  */
 #include <time.h>
-#include <sys/syscall.h>
+extern int nsi_host_clock_gettime(int clk_id, struct timespec *tp);
 static inline uint64_t bench_ns(void)
 {
 	struct timespec ts;
-	syscall(__NR_clock_gettime, CLOCK_MONOTONIC, &ts);
+	nsi_host_clock_gettime(CLOCK_MONOTONIC, &ts);
 	return (uint64_t)ts.tv_sec * 1000000000ULL + (uint64_t)ts.tv_nsec;
 }
 #else

tests/benchmarks/pid_benchmark/src/main.c

@@ -23,11 +23,11 @@
 
 #ifdef CONFIG_NATIVE_SIMULATOR
 #include <time.h>
-#include <sys/syscall.h>
+extern int nsi_host_clock_gettime(int clk_id, struct timespec *tp);
 static inline uint64_t bench_ns(void)
 {
 	struct timespec ts;
-	syscall(__NR_clock_gettime, CLOCK_MONOTONIC, &ts);
+	nsi_host_clock_gettime(CLOCK_MONOTONIC, &ts);
 	return (uint64_t)ts.tv_sec * 1000000000ULL + (uint64_t)ts.tv_nsec;
 }
 #else