rustc: Don't link in syntax extensions

This bug was introduced in #13384 by accident, and this commit continues the
work of #13384 by finishing support for loading a syntax extension crate without
registering it with the local cstore.

Closes #13495
auto merge of #13496 : alexcrichton/rust/issue-13495, r=sfackler

This bug was introduced in #13384 by accident, and this commit continues the
work of #13384 by finishing support for loading a syntax extension crate without
registering it with the local cstore.

Closes #13495