ART: More warnings

Enable -Wno-conversion-null, -Wredundant-decls and -Wshadow in general,
and -Wunused-but-set-parameter for GCC builds.

Change-Id: I81bbdd762213444673c65d85edae594a523836e5
diff --git a/runtime/arch/memcmp16.cc b/runtime/arch/memcmp16.cc
index 5a3e73e..813df2f 100644
--- a/runtime/arch/memcmp16.cc
+++ b/runtime/arch/memcmp16.cc
@@ -19,6 +19,7 @@
 // This linked against by assembly stubs, only.
 #pragma GCC diagnostic ignored "-Wunused-function"
 
+int32_t memcmp16_generic_static(const uint16_t* s0, const uint16_t* s1, size_t count);
 int32_t memcmp16_generic_static(const uint16_t* s0, const uint16_t* s1, size_t count) {
   for (size_t i = 0; i < count; i++) {
     if (s0[i] != s1[i]) {