Reuse CrateNum for proc-macro crates even when cross-compiling

Proc-macros are always compiled for the host, so this should be the same
in every way as recompiling the crate.

I am not sure why the previous code special-cased the target, since the
compiler properly gives an error when trying to load a crate for a
different host:

```
error[E0461]: couldn't find crate `dependency` with expected target triple x86_64-unknown-linux-gnu
  --> /home/joshua/rustc4/src/test/ui/cfg-dependent.rs:8:2
   |
LL |     dependency::is_64();
   |     ^^^^^^^^^^
   |
   = note: the following crate versions were found:
           crate `dependency`, target triple i686-unknown-linux-gnu: /home/joshua/rustc4/build/x86_64-unknown-linux-gnu/test/ui/cfg-dependent/auxiliary/libdependency.so
```

I think another possible fix is to remove the check altogether. But I'm
not sure, and this fix works, so I'm not making the larger change here.

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