Header refactoring: unify spin.h and move it out of the catch-all.

include/jemalloc/internal/spin.h

@@ -0,0 +1,36 @@
+#ifndef JEMALLOC_INTERNAL_SPIN_H
+#define JEMALLOC_INTERNAL_SPIN_H
+
+#ifdef JEMALLOC_SPIN_C_
+#  define SPIN_INLINE extern inline
+#else
+#  define SPIN_INLINE inline
+#endif
+
+#define SPIN_INITIALIZER {0U}
+
+typedef struct {
+	unsigned iteration;
+} spin_t;
+
+SPIN_INLINE void
+spin_adaptive(spin_t *spin) {
+	volatile uint32_t i;
+
+	if (spin->iteration < 5) {
+		for (i = 0; i < (1U << spin->iteration); i++) {
+			CPU_SPINWAIT;
+		}
+		spin->iteration++;
+	} else {
+#ifdef _WIN32
+		SwitchToThread();
+#else
+		sched_yield();
+#endif
+	}
+}
+
+#undef SPIN_INLINE
+
+#endif /* JEMALLOC_INTERNAL_SPIN_H */