cpu: Have "cpu" feature take precedence over "custom"

Right now, features are always enabled across all targets. This means
that if _any_ Custom RNG is used _anywhere_ in a crate's dependancy
graph on _any_ target. The "custom" feature will be active on all
targets.

This makes it impossible to have a bare metal target that uses "cpu" on
x86_64 and a Custom RNG on aarch64. This solution also makes sure that
any implementation `getrandom` itself provides cannot be overridden.

Signed-off-by: Joe Richey <[email protected]>

Author: josephlr

src/lib.rs

@@ -183,11 +183,11 @@ cfg_if! {
         #[path = "windows.rs"] mod imp;
     } else if #[cfg(all(target_arch = "x86_64", target_env = "sgx"))] {
         #[path = "rdrand.rs"] mod imp;
-    } else if #[cfg(feature = "custom")] {
-        use custom as imp;
     } else if #[cfg(all(feature = "cpu",
                         any(target_arch = "x86_64", target_arch = "x86")))] {
         #[path = "rdrand.rs"] mod imp;
+    } else if #[cfg(feature = "custom")] {
+        use custom as imp;
     } else {
         compile_error!("\
             target is not supported, for more information see: \