Fix linkage type for objc_method_cache_version.

This was accidentally common linkage, so ended up with copy relocations.

dtable.c

@@ -43,6 +43,8 @@ PRIVATE mutex_t initialize_lock;
  * 2^x in increments of 8. */
 static uint32_t dtable_depth = 8;
 
+_Atomic(uint64_t) objc_method_cache_version;
+
 /**
  * Starting at `cls`, finds the class that provides the implementation of the
  * method identified by `sel`.

objc/slot.h

@@ -6,32 +6,30 @@
 #define __OBJC_SLOT_H_INCLUDED__
 /**
  * The objc_slot structure is used to permit safe IMP caching.  It is returned
- * by the new lookup APIs.  When you cache an IMP, you should store a copy of
- * the version field and a pointer to the slot. 
- *
- * The slot version is guaranteed never to be 0.  When updating a cache, you
- * should use code of the following form:
- *
- * 1) version = 0;
- * 2) slot->cachedFor = receiver->isa;
- * 3) slot_cache = slot;
- * 4) version = slot->version;
- *
- * The runtime guarantees that the version in any cachable slot will never be
- * 0.  This should ensure that, if the version and cache pointer mismatch, the
- * next access will cause a cache miss.  
- *
- * When using a cached slot, you should compare the owner pointer to the isa
- * pointer of the receiver and the message and the version of the slot to your
- * cached version.
+ * by the new lookup APIs.  When you call `objc_slot_lookup_version`, the final
+ * parameter is used to return either the current value of
+ * `objc_method_cache_version` or 0 if the slot is uncacheable.  You can then
+ * store this value along with a pointer to the `objc_slot2`.  If the returned
+ * value is equal to the current value of `objc_method_cache_version` then it
+ * is safe to call the method from the `method` field of the slot directly.
  */
 struct objc_slot2
 {
 	IMP method;
 } OBJC_NONPORTABLE;
 
-_Atomic(uint64_t) objc_method_cache_version;
+/**
+ * A counter that is incremented whenever one or more cached slots become
+ * invalid, for example if a subclass loads a category containing methods that
+ * were inherited from the superclass.
+ */
+PUBLIC _Atomic(uint64_t) objc_method_cache_version;
 
+/**
+ * Legacy cache structure.  This is no longer maintained in the runtime and is
+ * now exported only in the compatibility interfaces.  Slots of this form are
+ * never cacheable.
+ */
 struct objc_slot
 {
 	/** The class to which this slot is attached (used internally).  */