support for mips32r6 as a target_arch value

Author: chenx97

example/alloc_system.rs

@@ -10,6 +10,7 @@
 #[cfg(any(target_arch = "x86",
               target_arch = "arm",
               target_arch = "mips",
+              target_arch = "mips32r6",
               target_arch = "powerpc",
               target_arch = "powerpc64"))]
 const MIN_ALIGN: usize = 8;