Fix missing macOS slice

This adds the `S_ATTR_NO_DEAD_STRIP` section attribute, which forces
"unused" code to remain in the binary. This is paired with `S_REGULAR`
to provide a required section type.

This fixes dtolnay#41.

impl/src/linker.rs

@@ -34,7 +34,7 @@ pub mod macos {
     use syn::Ident;
 
     pub fn section(ident: &Ident) -> String {
-        format!("__DATA,__linkme{}", crate::hash(ident))
+        format!("__DATA,__linkme{},regular,no_dead_strip", crate::hash(ident))
     }
 
     pub fn section_start(ident: &Ident) -> String {