Add missing comma in alloc_system

Author: antoyo

example/alloc_system.rs

@@ -12,7 +12,7 @@
               target_arch = "mips",
               target_arch = "mips32r6",
               target_arch = "powerpc",
-              target_arch = "csky"
+              target_arch = "csky",
               target_arch = "powerpc64"))]
 const MIN_ALIGN: usize = 8;
 #[cfg(any(target_arch = "x86_64",