Force clang for bionic

Bionic will only compile with clang, and Android.mk files force clang,
turn it on in Android.bp.

Change-Id: I9db06298adfeab4eeef031d88aa310776cad3af0
diff --git a/libc/Android.bp b/libc/Android.bp
index 2c393c3..4ffe72d 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -76,6 +76,7 @@
         never: true,
     },
     native_coverage: false,
+    clang: true,
 }
 
 // ANDROIDMK TRANSLATION ERROR: unsupported directive