libstd/rand/os.rs: Remove a tiny bit of duplicated code

It's nearly midnight. I'm tired. I'll look for something worth doing in the morning :)

src/libstd/rand/os.rs

@@ -41,9 +41,7 @@ fn getrandom(buf: &mut [u8]) -> libc::c_long {
         const NR_GETRANDOM: libc::c_long = 318;
         #[cfg(target_arch = "x86")]
         const NR_GETRANDOM: libc::c_long = 355;
-        #[cfg(any(target_arch = "arm", target_arch = "aarch64"))]
-        const NR_GETRANDOM: libc::c_long = 384;
-        #[cfg(target_arch = "powerpc")]
+        #[cfg(any(target_arch = "arm", target_arch = "aarch64", target_arch = "powerpc"))]
         const NR_GETRANDOM: libc::c_long = 384;
 
         unsafe {