Native: add ExperimentalForeignApi opt-in to performance tests

3f3f6eb marks all cinterop-generated declarations with
@ExperimentalForeignApi. As a result, all usages of such declarations
have to opt-in explicitly. This commit adds those opt-ins to all
usages in the performance tests.

^KT-58362

kotlin-native/performance/numerical/src/main/kotlin-native/launcher.kt

@@ -22,6 +22,7 @@ fun konanBellardPi() {
 
 fun clangBellardPi() {
     for (n in 1 .. 1000 step 9) {
+        @OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
         val result = cinterop.pi_nth_digit(n)
         Blackhole.consume(result)
     }