add AOS_SANITIZER_*

Change-Id: I69c35724be636eec2ccbcc93ed8b7bc01e3bebd1
diff --git a/aos/build/aos.gypi b/aos/build/aos.gypi
index dca03b3..c390052 100644
--- a/aos/build/aos.gypi
+++ b/aos/build/aos.gypi
@@ -91,6 +91,10 @@
           'ldflags': [
             '-fsanitize=<(SANITIZER)',
           ],
+          'defines': [
+# GCC doesn't have __has_feature, so we have to use this instead.
+            'AOS_SANITIZER_<(SANITIZER)',
+          ],
         },
       },
     ], ['SANITIZER!="none" and COMPILER!="gcc"', {