Add atomic swap and cmpxchg for void * as well.

Issue #232.

docs/ispc.rst

@@ -3797,6 +3797,13 @@ For global atomics, only atomic swap is available for these types:
   float atomic_swap_global(uniform float * uniform ptr, float value)
   double atomic_swap_global(uniform double * uniform ptr, double value)
 
+Finally, "swap" (but none of these other atomics) is available for pointer
+types:
+
+::
+
+  void *atomic_swap_{local,global}(void * * uniform ptr, void * value)
+
 There are also variants of the atomic that take ``uniform`` values for the
 operand and return a ``uniform`` result.  These correspond to a single
 atomic operation being performed for the entire gang of program instances,
@@ -3821,6 +3828,13 @@ rather than one per program instance.
   uniform int32 atomic_swap_{local,global}(uniform int32 * uniform ptr,
                                            uniform int32 newval)
 
+And similarly for pointers:
+
+::
+
+  uniform void *atomic_swap_{local,global}(void * * uniform ptr,
+                                           void *newval)
+
 Be careful that you use the atomic function that you mean to; consider the
 following code:
 
@@ -3862,12 +3876,18 @@ the same location in memory!)
   int32 atomic_xor_{local,global}(uniform int32 * varying ptr, int32 value)
   int32 atomic_swap_{local,global}(uniform int32 * varying ptr, int32 value)
 
+And:
+
+::
+
+  void *atomic_swap_{local,global}(void * * ptr, void *value)
+
 There are also atomic "compare and exchange" functions.  Compare and
 exchange atomically compares the value in "val" to "compare"--if they
 match, it assigns "newval" to "val".  In either case, the old value of
 "val" is returned.  (As with the other atomic operations, there are also
 ``unsigned`` and 64-bit variants of this function.  Furthermore, there are
-``float`` and ``double`` variants as well.)
+``float``, ``double``, and ``void *`` variants as well.)
 
 ::