Don't use SSE4.1 instructions unless the target supports it

Fixes servo#48.

Enabling the SIMD code path now requires building with
`RUSTFLAGS="-C target-feature=+sse4.1"` or similar.

src/lib.rs

@@ -7,7 +7,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![cfg_attr(feature = "unstable", feature(asm, repr_simd, test))]
+#![cfg_attr(feature = "unstable", feature(asm, cfg_target_feature, repr_simd, test))]
 
 extern crate heapsize;
 

src/side_offsets.rs

@@ -146,13 +146,13 @@ impl SideOffsets2DSimdI32 {
         }
     }
 
-    #[cfg(not(target_arch = "x86_64"))]
+    #[cfg(not(target_feature = "sse4.1"))]
     #[inline]
     pub fn is_zero(&self) -> bool {
         self.top == 0 && self.right == 0 && self.bottom == 0 && self.left == 0
     }
 
-    #[cfg(target_arch = "x86_64")]
+    #[cfg(target_feature = "sse4.1")]
     #[inline]
     pub fn is_zero(&self) -> bool {
         let is_zero: bool;