cpu: Add support for CPU-based randomness

This allows freestanding targets to use getrandom.

Signed-off-by: Joe Richey <joerichey@google.com>

Cargo.toml

@@ -35,6 +35,8 @@ wasi = "0.9"
 
 [features]
 std = []
+# Feature to enable fallback CPU-based implementation
+cpu = []
 # Feature to enable custom RNG implementations
 custom = []
 # Unstable feature to support being a libstd dependency

src/lib.rs

@@ -185,6 +185,8 @@ cfg_if! {
         #[path = "rdrand.rs"] mod imp;
     } else if #[cfg(feature = "custom")] {
         use custom as imp;
+    } else if #[cfg(all(feature = "cpu", target_arch = "x86_64"))] {
+        #[path = "rdrand.rs"] mod imp;
     } else {
         compile_error!("\
             target is not supported, for more information see: \