Use -Werror in system/core

* Move -Wall -Werror from cppflags to cflags.
* Fix/suppress warning on unused variables.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I1e05e96a1d0bcb2ccef1ce456504b3af57167cc5
diff --git a/libvndksupport/Android.bp b/libvndksupport/Android.bp
index b624223..fec79b7 100644
--- a/libvndksupport/Android.bp
+++ b/libvndksupport/Android.bp
@@ -3,6 +3,7 @@
 cc_library {
     name: "libvndksupport",
     srcs: ["linker.c"],
+    cflags: ["-Wall", "-Werror"],
     local_include_dirs: ["include/vndksupport"],
     export_include_dirs: ["include"],
     shared_libs: ["liblog"],