checker: allow for v -is_o -o x mm.v, where mm.v starts with module abc (i.e. not just module main)

vlib/v/checker/checker.v

@@ -458,6 +458,10 @@ pub fn (mut c Checker) check_files(ast_files []&ast.File) {
 		// shared libs do not need to have a main
 		return
 	}
+	if c.pref.is_o {
+		// .o files also do not need main
+		return
+	}
 	if c.pref.no_builtin {
 		// `v -no-builtin module/` do not necessarily need to have a `main` function
 		// This is useful for compiling linux kernel modules for example.