build: build v8 with -fvisibility=hidden

V8 should be built with -fvisibility=hidden, otherwise
the resulting binary would contain unnecessary symbols. In
particular, on macOS, this leads to 5000+ weak symbols
resolved at runtime, leading to a startup regression.

On macOS this also reduces the binary size about ~10MB.
On Linux the size reduction is around 8MB.

tools/v8_gypfiles/v8.gyp

@@ -41,6 +41,10 @@
         'AdditionalOptions': ['/utf-8']
       }
     },
+    'cflags': ['-fvisibility=hidden'],
+    'xcode_settings': {
+      'GCC_SYMBOLS_PRIVATE_EXTERN': 'YES',  # -fvisibility=hidden
+    },
   },
   'targets': [
     {