Basic support for AVX2 when building with LLVM3.1svn

For now this target just uses the same builtins-*.ll files as the
regular AVX1 target.  Once the gather intrinsic is available from
LLVM, we'll want to have custom target files that call out to that
for gathers. (The integer min/max intrinsics should be wired up to
the __{min,max}_varying_{int,uint}*() builtins at that point as
well.)

module.cpp

@@ -1169,6 +1169,9 @@ Module::execPreprocessor(const char* infilename, llvm::raw_string_ostream* ostre
     case Target::AVX:
         opts.addMacroDef("ISPC_TARGET_AVX");
         break;
+    case Target::AVX2:
+        opts.addMacroDef("ISPC_TARGET_AVX2");
+        break;
     default:
         FATAL("Unhandled target ISA in preprocessor symbol definition");
     }