Add getentropy for RTEMS

RTEMS provides the getentropy function.
Use this for providing random data.

Author: thesummer

library/std/src/sys/random/mod.rs

@@ -22,7 +22,7 @@ cfg_if::cfg_if! {
     ))] {
         mod arc4random;
         pub use arc4random::fill_bytes;
-    } else if #[cfg(target_os = "emscripten")] {
+    } else if #[cfg(any(target_os = "emscripten", target_os = "rtems"))] {
         mod getentropy;
         pub use getentropy::fill_bytes;
     } else if #[cfg(target_os = "espidf")] {