markused, builtin: apply @[markused] to builtin_init, remove its heuristic from markused.v

vlib/builtin/builtin_nix.c.v

@@ -3,6 +3,7 @@
 // that can be found in the LICENSE file.
 module builtin
 
+@[markused]
 fn builtin_init() {
 	$if gcboehm ? {
 		$if !gc_warn_on_stderr ? {

vlib/builtin/builtin_windows.c.v

@@ -63,6 +63,7 @@ const enable_processed_output = 1
 const enable_wrap_at_eol_output = 2
 const evable_virtual_terminal_processing = 4
 
+@[markused]
 fn builtin_init() {
 	$if gcboehm ? {
 		$if !gc_warn_on_stderr ? {

vlib/v/markused/markused.v

@@ -60,9 +60,6 @@ pub fn mark_used(mut table ast.Table, mut pref_ pref.Preferences, ast_files []&a
 		if pref_.is_bare {
 			core_fns << 'init_global_allocator' // needed for linux_bare and wasm_bare
 		}
-		if ast_files[ast_files.len - 1].imports.len > 0 {
-			core_fns << 'builtin_init'
-		}
 		if 'use_libbacktrace' in pref_.compile_defines {
 			core_fns << 'print_libbacktrace'
 		}