Dwarf: cleanup emitted debug info

* reduce iteration cost by not tracking unused entries
 * avoid emitting unused abbrevs to `.debug_abbrev`
 * get the compiler executable passing `llvm-dwarfdump --verify`
 * make it possible to skip `.debug_line` padding much more quickly

lib/std/dwarf.zig

@@ -89,6 +89,7 @@ pub const LNS = struct {
 };
 
 pub const LNE = struct {
+    pub const padding = 0x00;
     pub const end_sequence = 0x01;
     pub const set_address = 0x02;
     pub const define_file = 0x03;