Remove force-host and explain no-prefer-dynamic

tests/ui/crashes/auxiliary/proc_macro_crash.rs

@@ -1,5 +1,8 @@
-// force-host
 // 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
+// compile this as dylib. Removing this then causes the test to fail because a `dylib` crate can't
+// contain a proc-macro.
 
 #![feature(repr128)]
 #![crate_type = "proc-macro"]