version script: support `extern "C++"`

[marxin]

While building the Gnome distribution on Gentoo, I noticed there are a couple of packages that rely on demangled names in their version scripts:
https://github.com/google/highway/blob/0e759dd39119644c902eb0dcda2cfe03483c40ac/hwy/hwy.version#L4

Specification: https://sourceware.org/binutils/docs/ld/VERSION.html

[marxin]

webkit-gtk gentoo package example:

{
global:
  jsc_*;
  webkit_*;
  extern "C++" {
    "WebKit::NetworkProcessMain(int, char**)";
    "WebKit::WebProcessExtensionManager::initialize(WebKit::InjectedBundle*, API::Object*)";
    "WebKit::WebProcessExtensionManager::singleton()";
    "WebKit::WebProcessMain(int, char**)";
  };
local:
  *;
};