rustc: Forbid plugin_registrar in only rlib form

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475
auto merge of #15565 : alexcrichton/rust/issue-15475, r=huonw

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475