Use aligned cmsghdr structs test_cmsg_nxthdr

Author: ChrisDenton

libc-test/test/cmsg.rs

@@ -57,15 +57,15 @@ mod t {
     #[test]
     // FIXME: This triggers alignment checks for pointer dereferences:
     // https://github.com/rust-lang/libc/issues/3181
-    #[ignore]
     fn test_cmsg_nxthdr() {
         use std::ptr;
 
-        let mut buffer = [0u8; 256];
+        const CAPACITY: usize = 64 + 196 / mem::size_of::<usize>();
+        let mut buffer = [0_usize; CAPACITY];
         let mut mhdr: msghdr = unsafe { mem::zeroed() };
         let pmhdr = &mhdr as *const msghdr;
         for start_ofs in 0..64 {
-            let pcmsghdr = &mut buffer[start_ofs] as *mut u8 as *mut cmsghdr;
+            let pcmsghdr = &mut buffer[start_ofs] as *mut usize as *mut cmsghdr;
             mhdr.msg_control = pcmsghdr as *mut c_void;
             mhdr.msg_controllen = (160 - start_ofs) as _;
             for cmsg_len in 0..64 {