unix: don't explicitly instantiate v8::Persistent<x> templates
unix: don't explicitly instantiate v8::Persistent<x> templates

These explicit instantiations were added to make MSVC happy. It turns
out that some older versions of gcc and llvm now complain about duplicate
symbols, so we instantiate these templates only when MSVC is used.

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node