Allow some bionic tests to compile with gcc.

bionic/tests has some test cases compiled with both gcc and clang.

Bug: 30575506
Change-Id: I68576a931021089ecc6b9c9d14ef0b3e3cf05e2d
Test: build and find new warning in build.log
diff --git a/core/clang/config.mk b/core/clang/config.mk
index e47c36a..dbdfa61 100644
--- a/core/clang/config.mk
+++ b/core/clang/config.mk
@@ -145,6 +145,7 @@
 
 # A list of projects that are allowed to set LOCAL_CLANG to false.
 LOCAL_CLANG_EXCEPTION_PROJECTS := \
+  bionic/tests/ \
   device/huawei/angler/ \
   device/lge/bullhead/ \
   external/valgrind/ \