build: compile memory intrinsics on xous

Like SGX, Xous does not have any libc to link against. As a result,
memory intrinsics need to be available as part of `compiler_builtins`

Signed-off-by: Sean Cross <sean@xobs.io>

library/compiler-builtins/build.rs

@@ -30,6 +30,7 @@ fn main() {
         || target.contains("-none")
         || target.contains("nvptx")
         || target.contains("uefi")
+        || target.contains("xous")
     {
         println!("cargo:rustc-cfg=feature=\"mem\"");
     }