fix empty entry in gcc cmd (#1129)

op_builder/async_io.py

@@ -30,7 +30,7 @@ class AsyncIOBuilder(OpBuilder):
         return ['csrc/aio/py_lib', 'csrc/aio/common']
 
     def cxx_args(self):
-        return [
+        args = [
             '-g',
             '-Wall',
             '-O0',
@@ -41,9 +41,14 @@ class AsyncIOBuilder(OpBuilder):
             '-march=native',
             '-fopenmp',
             '-laio',
-            self.simd_width()
         ]
 
+        simd_width = self.simd_width()
+        if len(simd_width) > 0:
+            args.append(simd_width)
+
+        return args
+
     def extra_ldflags(self):
         return ['-laio']