auto merge of #5943 : brson/rust/struct-return, r=graydon

Mac appears to follow the same ABI as MSVC. This fixes one case
but leaves others broken, like windows.

src/test/run-pass/extern-pass-TwoU32s.rs

@@ -11,8 +11,6 @@
 // Test a foreign function that accepts and returns a struct
 // by value.
 
-// xfail-macos Broken on mac i686
-
 #[deriving(Eq)]
 struct TwoU32s {
     one: u32, two: u32

src/test/run-pass/extern-return-TwoU32s.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// xfail-macos Broken on mac i686
-
 struct TwoU32s {
     one: u32, two: u32
 }