Use emit=link for auxiliary proc macro crates

tests/ui/auxiliary/proc_macro_attr.rs

@@ -1,3 +1,4 @@
+// compile-flags: --emit=link
 // no-prefer-dynamic
 
 #![crate_type = "proc-macro"]

tests/ui/auxiliary/proc_macro_derive.rs

@@ -1,3 +1,4 @@
+// compile-flags: --emit=link
 // no-prefer-dynamic
 
 #![crate_type = "proc-macro"]

tests/ui/crashes/auxiliary/proc_macro_crash.rs

@@ -1,3 +1,4 @@
+// compile-flags: --emit=link
 // no-prefer-dynamic
 // ^ compiletest by default builds all aux files as dylibs, but we don't want that for proc-macro
 // crates. If we don't set this, compiletest will override the `crate_type` attribute below and