RESTRICT AUTOMERGE Ignore weak symbol difference in libc++ ABI check

The diff_flags make the ABI checker ignore the weak symbol difference
caused by PGO.

Test: make libc++.vendor
Bug: 230076879
Change-Id: Ifff053fd9a1411cbe73ab48359ffa6f49bec4490

Android.bp

@@ -180,6 +180,10 @@ cc_library_shared {
             enabled: true,
         },
     },
+
+    header_abi_checker: {
+        diff_flags: ["-allow-adding-removing-weak-symbols"],
+    },
 }
 
 cc_library_static {