Fix aarch64 build issue with pthread_mutexattr_t alignment change

aarch64 pthread_mutexattr_t requires 8 byte alignment unlike
other 64bit architectures, so we can't use target_pointer_width.

Author: antonblanchard

src/unix/notbsd/linux/mod.rs

@@ -83,9 +83,11 @@ s! {
     }
 
     pub struct pthread_mutexattr_t {
-        #[cfg(target_pointer_width = "64")]
+        #[cfg(any(target_arch = "x86_64", target_arch = "powerpc64",
+                  target_arch = "powerpc64le"))]
         __align: [::c_int; 0],
-        #[cfg(target_pointer_width = "32")]
+        #[cfg(not(any(target_arch = "x86_64", target_arch = "powerpc64",
+                      target_arch = "powerpc64le")))]
         __align: [::c_long; 0],
         size: [u8; __SIZEOF_PTHREAD_MUTEXATTR_T],
     }