rustc_data_structures: Fix copy-and-paste error from previous change

This fixes a trivial copy-and-paste error from the previous change
which accidentally listed "powerpc" twice instead of "mips" and
"powerpc" in the architecture list for using portable AtomicU64.

Author: glaubitz

compiler/rustc_data_structures/src/marker.rs

@@ -149,7 +149,7 @@ cfg_match! {
 
         // MIPS, PowerPC and SPARC platforms with 32-bit pointers do not
         // have AtomicU64 type.
-        #[cfg(not(any(target_arch = "powerpc", target_arch = "powerpc", target_arch = "sparc")))]
+        #[cfg(not(any(target_arch = "mips", target_arch = "powerpc", target_arch = "sparc")))]
         already_sync!(
             [std::sync::atomic::AtomicU64]
         );