fix oom2 test

Author: RalfJung

tests/compile-fail/oom2.rs

@@ -1,10 +1,14 @@
 // Validation forces more allocation; disable it.
 // compile-flags: -Zmir-emit-validate=0
 #![feature(box_syntax, custom_attribute, attr_literals)]
-#![miri(memory_size=2048)]
+#![miri(memory_size=1024)]
+
+// On 64bit platforms, the allocator needs 32 bytes allocated to pass a return value, so that's the error we see.
+// On 32bit platforms, it's just 16 bytes.
+// error-pattern: tried to allocate
 
 fn main() {
     loop {
-        ::std::mem::forget(box 42); //~ ERROR tried to allocate 4 more bytes
+        ::std::mem::forget(box 42);
     }
 }