allow #[rustc_align_static(N)] on statics

We need a different attribute than `rustc_align` because unstable attributes are
tied to their feature (we can't have two unstable features use the same
unstable attribute). Otherwise this uses all of the same infrastructure
as `#[rustc_align]`.

compiler/rustc_codegen_gcc/src/consts.rs

@@ -81,6 +81,8 @@ impl<'gcc, 'tcx> StaticCodegenMethods for CodegenCx<'gcc, 'tcx> {
         if global.to_rvalue().get_type() != val_llty {
             global.to_rvalue().set_type(val_llty);
         }
+
+        // NOTE: Alignment from attributes has already been applied to the allocation.
         set_global_alignment(self, global, alloc.align);
 
         global.global_set_initializer_rvalue(value);