The previous approach (out-of-line member definitions plus explicit
template instantiations) still produces weak_odr vtables, which Mach-O
ThinLTO localizes during code generation even when other LTO partitions
reference them. This breaks the link of executables that link libnode
(electron_natives_codecache, node_mksnapshot) with undefined vtable
symbol errors.

Replace that part of the node patch with explicit specializations of
the affected instantiations:

  - StaticExternalByteResource<uint8_t, char, ExternalOneByteStringResource>
  - StaticExternalByteResource<uint16_t, uint16_t, ExternalStringResource>
  - NgRcBufPointer<Http2RcBufferPointerTraits>::External

An explicit specialization is an ordinary class that can have a key
function (an out-of-line destructor), so its vtable is a strong symbol
that LTO must keep external. The primary templates and all user code
are unchanged, and no linker or compiler optimizations are disabled.

The non-template fixes (MemoryRetainerNode, JSGraphJSNode key
functions) and the N-API __attribute__((used)) changes are kept as-is.

Co-authored-by: Samuel Attard <sam@electronjs.org>