Ignore new compiler warnings

Clang 6.0.1 update introduced a number of new warnings, failing
werror build. Ignore the -Wvisibility, -Wincompatible-pointer-types,
-Wformat warnings for external/compiler-rt.

Test: m checkbuild
Change-Id: I56ac95dbea43012949786625952d9a395addae84
diff --git a/Android.bp b/Android.bp
index a73b7ec..d88e736 100644
--- a/Android.bp
+++ b/Android.bp
@@ -36,6 +36,8 @@
 
     cflags: [
         "-Wno-unused-parameter",
+        "-Wno-visibility",
+        "-Wno-incompatible-pointer-types",
         "-Werror",
     ],
 
@@ -234,6 +236,8 @@
 
     cflags: [
         "-Wno-unused-parameter",
+        "-Wno-visibility",
+        "-Wno-incompatible-pointer-types",
         "-Werror",
     ],