add #[align] attribute

Right now it's used for functions with `fn_align`, in the future it will
get more uses (statics, struct fields, etc.)

Author: folkertdev

tests/pass/fn_align.rs

@@ -1,21 +1,21 @@
 //@compile-flags: -Zmin-function-alignment=8
 #![feature(fn_align)]
 
-// When a function uses `repr(align(N))`, the function address should be a multiple of `N`.
+// When a function uses `align(N)`, the function address should be a multiple of `N`.
 
-#[repr(align(256))]
+#[align(256)]
 fn foo() {}
 
-#[repr(align(16))]
+#[align(16)]
 fn bar() {}
 
-#[repr(align(4))]
+#[align(4)]
 fn baz() {}
 
 fn main() {
     assert!((foo as usize).is_multiple_of(256));
     assert!((bar as usize).is_multiple_of(16));
 
-    // The maximum of `repr(align(N))` and `-Zmin-function-alignment=N` is used.
+    // The maximum of `align(N)` and `-Zmin-function-alignment=N` is used.
     assert!((baz as usize).is_multiple_of(8));
 }