make compile-core: fix warnings

It is easier to spot real problems if the code is warning-free.

Replacing `gensym` with `make-symbol` is an idea taken from here:
b44c08dd45

In defer-until!:
core-lib.el:150:19:Warning: function ‘gensym’ from cl package called at
    runtime

In add-transient-hook!:
core-lib.el:216:16:Warning: function ‘gensym’ from cl package called at
    runtime

In toplevel form:
autoload/message.el:35:1:Warning: Unused lexical variable ‘spec’

In toplevel form:
autoload/line-numbers.el:31:1:Warning: defcustom for
    ‘display-line-numbers-type’ fails to specify containing group
autoload/line-numbers.el:31:1:Warning: defcustom for
    ‘display-line-numbers-type’ fails to specify containing group
autoload/line-numbers.el:39:1:Warning: defcustom for
    ‘display-line-numbers-grow-only’ fails to specify containing group
autoload/line-numbers.el:39:1:Warning: defcustom for
    ‘display-line-numbers-grow-only’ fails to specify containing group
autoload/line-numbers.el:44:1:Warning: defcustom for
    ‘display-line-numbers-width-start’ fails to specify containing group
autoload/line-numbers.el:44:1:Warning: defcustom for
    ‘display-line-numbers-width-start’ fails to specify containing group

In toplevel form:
cli/autoloads.el:137:1:Warning: Unused lexical variable ‘type’

Preserve name of unused lexical var _type

Makes it obvious what is stored there.

core/cli/autoloads.el

@@ -192,7 +192,7 @@ even if it doesn't need reloading!"
                         (push `(put ',name 'doom-module ',origin) forms))))
 
                    ((eq type 'defalias)
-                    (cl-destructuring-bind (type name target &optional docstring) sexp
+                    (cl-destructuring-bind (_type name target &optional docstring) sexp
                       (let ((name (doom-unquote name))
                             (target (doom-unquote target)))
                         (unless (and member-p