atomic: Remove atomic_forced_read

Remove the odd atomic_forced_read which is neither atomic nor forced.
Some uses are completely redundant, so simply remove them.  In other cases
the intended use is to force a memory ordering, so use acquire load for those.
In yet other cases their purpose is unclear, for example __nscd_cache_search
appears to allow concurrent accesses to the cache while it is being garbage
collected by another thread!  Use relaxed atomic loads here to block spills
from accidentally reloading memory that is being changed.

Passes regress on AArch64, OK for commit?

malloc/malloc-debug.c

@@ -155,7 +155,7 @@ static size_t pagesize;
 static void *
 __debug_malloc (size_t bytes)
 {
-  void *(*hook) (size_t, const void *) = atomic_forced_read (__malloc_hook);
+  void *(*hook) (size_t, const void *) = __malloc_hook;
   if (__glibc_unlikely (hook != NULL))
     return (*hook)(bytes, RETURN_ADDRESS (0));
 
@@ -179,7 +179,7 @@ strong_alias (__debug_malloc, malloc)
 static void
 __debug_free (void *mem)
 {
-  void (*hook) (void *, const void *) = atomic_forced_read (__free_hook);
+  void (*hook) (void *, const void *) = __free_hook;
   if (__glibc_unlikely (hook != NULL))
     {
       (*hook)(mem, RETURN_ADDRESS (0));
@@ -201,8 +201,7 @@ strong_alias (__debug_free, free)
 static void *
 __debug_realloc (void *oldmem, size_t bytes)
 {
-  void *(*hook) (void *, size_t, const void *) =
-    atomic_forced_read (__realloc_hook);
+  void *(*hook) (void *, size_t, const void *) = __realloc_hook;
   if (__glibc_unlikely (hook != NULL))
     return (*hook)(oldmem, bytes, RETURN_ADDRESS (0));
 
@@ -230,8 +229,7 @@ strong_alias (__debug_realloc, realloc)
 static void *
 _debug_mid_memalign (size_t alignment, size_t bytes, const void *address)
 {
-  void *(*hook) (size_t, size_t, const void *) =
-    atomic_forced_read (__memalign_hook);
+  void *(*hook) (size_t, size_t, const void *) = __memalign_hook;
   if (__glibc_unlikely (hook != NULL))
     return (*hook)(alignment, bytes, address);
 
@@ -330,7 +328,7 @@ __debug_calloc (size_t nmemb, size_t size)
       return NULL;
     }
 
-  void *(*hook) (size_t, const void *) = atomic_forced_read (__malloc_hook);
+  void *(*hook) (size_t, const void *) = __malloc_hook;
   if (__glibc_unlikely (hook != NULL))
     {
       void *mem = (*hook)(bytes, RETURN_ADDRESS (0));