run test only on 64bit

src/test/mir-opt/inline/inline-into-box-place.rs

@@ -1,6 +1,7 @@
 // ignore-tidy-linelength
 // ignore-wasm32-bare compiled with panic=abort by default
 // compile-flags: -Z mir-opt-level=3
+// only-64bit FIXME: the mir representation of RawVec depends on ptr size
 #![feature(box_syntax)]
 
 fn main() {