Implement global atomics and a memory barrier in the standard library.

This checkin provides the standard set of atomic operations and a memory barrier in the ispc standard library.  Both signed and unsigned 32- and 64-bit integer types are supported.

tests/atomics-5.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+uniform int32 s = 0xbeef;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = atomic_swap_global(s, programIndex);
+    RET[programIndex] = reduce_max(b);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0xbeef;
+}