fix proc-macro test added here to solely be exercised as a build product for the host.

thus we should no longer see test failures for e.g. wasm32 target.

tests/ui/proc-macro/auxiliary/exports_no_mangle.rs

@@ -1,3 +1,5 @@
+// force-host
+// no-prefer-dynamic
 #![crate_type="lib"]
 
 // Issue 111888: this crate (1.) is imported by a proc-macro crate and (2.)

tests/ui/proc-macro/no-mangle-in-proc-macro-issue-111888.rs

@@ -1,4 +1,6 @@
 // build-pass
+// force-host
+// no-prefer-dynamic
 // aux-build:exports_no_mangle.rs
 #![crate_type = "proc-macro"]