std: Use memalign, not posix_memalign, on Android

We've gotten requests to move our Android support as far back as API level 9
where unfortunately the `posix_memalign` API wasn't implemented yet. Thankfully,
however, the `memalign` API was and it appears to be usable with `free` on the
Android platform (see comments included in commit).

This should help fix some of the last few test failures when compiling against
API level 9.

src/liballoc_system/lib.rs

@@ -80,13 +80,40 @@ mod imp {
         if align <= MIN_ALIGN {
             libc::malloc(size as libc::size_t) as *mut u8
         } else {
-            let mut out = ptr::null_mut();
-            let ret = libc::posix_memalign(&mut out, align as libc::size_t, size as libc::size_t);
-            if ret != 0 {
-                ptr::null_mut()
-            } else {
-                out as *mut u8
-            }
+            aligned_malloc(size, align)
+        }
+    }
+
+    #[cfg(target_os = "android")]
+    unsafe fn aligned_malloc(size: usize, align: usize) -> *mut u8 {
+        // On android we currently target API level 9 which unfortunately
+        // doesn't have the `posix_memalign` API used below. Instead we use
+        // `memalign`, but this unfortunately has the property on some systems
+        // where the memory returned cannot be deallocated by `free`!
+        //
+        // Upon closer inspection, however, this appears to work just fine with
+        // Android, so for this platform we should be fine to call `memalign`
+        // (which is present in API level 9). Some helpful references could
+        // possibly be chromium using memalign [1], attempts at documenting that
+        // memalign + free is ok [2] [3], or the current source of chromium
+        // which still uses memalign on android [4].
+        //
+        // [1]: https://codereview.chromium.org/10796020/
+        // [2]: https://code.google.com/p/android/issues/detail?id=35391
+        // [3]: https://bugs.chromium.org/p/chromium/issues/detail?id=138579
+        // [4]: https://chromium.googlesource.com/chromium/src/base/+/master/
+        //                                       /memory/aligned_memory.cc
+        libc::memalign(align as libc::size_t, size as libc::size_t) as *mut u8
+    }
+
+    #[cfg(not(target_os = "android"))]
+    unsafe fn aligned_malloc(size: usize, align: usize) -> *mut u8 {
+        let mut out = ptr::null_mut();
+        let ret = libc::posix_memalign(&mut out, align as libc::size_t, size as libc::size_t);
+        if ret != 0 {
+            ptr::null_mut()
+        } else {
+            out as *mut u8
         }
     }