build: build v8 with -fvisibility=hidden

This enables -fvisibility=hidden on platforms other than macOS,
wherever it's supported.

Author: joyeecheung

tools/v8_gypfiles/v8.gyp

@@ -42,8 +42,7 @@
       }
     },
     # Hide symbols that are not explicitly exported with V8_EXPORT.
-    # TODO(joyeecheung): enable it on other platforms. Currently gcc times out
-    # or run out of memory with -fvisibility=hidden on some machines in the CI.
+    'cflags': ['-fvisibility=hidden'],
     'xcode_settings': {
       'GCC_SYMBOLS_PRIVATE_EXTERN': 'YES',  # -fvisibility=hidden
     },