Change libvixl symbol visibility.

Make all symbols protected for shared library and hidden for
static library. This reduces the size of the shared library
by ~44KiB on arm and ~78KiB on arm64, configured with both
arm and arm64 target. Binaries that link libvixl statically
are also a lot smaller, depending on the used libvixl parts.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 186902856
Change-Id: I5732117c4f2a564a032fa2642957efb63390035b
diff --git a/Android.bp b/Android.bp
index 3e714a9..23dd084 100644
--- a/Android.bp
+++ b/Android.bp
@@ -188,6 +188,17 @@
         "com.android.art",
         "com.android.art.debug",
     ],
+
+    static: {
+        cflags: [
+            "-fvisibility=hidden",
+        ],
+    },
+    shared: {
+        cflags: [
+            "-fvisibility=protected",
+        ],
+    },
 }
 
 art_cc_library {