rustc: Allow cdylibs to link against dylibs

Previously, rustc mandated that cdylibs could only link against rlibs as
dependencies (not dylibs).
This commit disables that restriction and tests that it works in a
simple case.

src/test/ui/auxiliary/cdylib-dep.rs

@@ -1 +0,0 @@
-#![crate_type = "dylib"]

src/test/ui/cdylib-deps-must-be-static.rs

@@ -1,10 +0,0 @@
-// build-fail
-// error-pattern: crate `cdylib_dep` required to be available in rlib format, but was not found
-// aux-build:cdylib-dep.rs
-// ignore-musl
-// ignore-cloudabi
-// ignore-emscripten
-// ignore-sgx no dynamic libraries
-#![crate_type = "cdylib"]
-
-extern crate cdylib_dep;

src/test/ui/cdylib-deps-must-be-static.stderr

@@ -1,4 +0,0 @@
-error: crate `cdylib_dep` required to be available in rlib format, but was not found in this form
-
-error: aborting due to previous error
-