Enhance spin_adaptive() to yield after several iterations.

This avoids worst case behavior if e.g. another thread is preempted
while owning the resource the spinning thread is waiting for.

test/unit/spin.c

@@ -0,0 +1,16 @@
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_spin) {
+	spin_t spinner = SPIN_INITIALIZER;
+
+	for (unsigned i = 0; i < 100; i++) {
+		spin_adaptive(&spinner);
+	}
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    test_spin);
+}