refactor(lib): use num-processors

This C function was introduced in Emacs 28.1.

lisp/lib/system.el

@@ -83,6 +83,8 @@
   "Return the max number of processing units on this system.
 Tries to be portable. Returns 1 if cannot be determined."
   (with-memoization (get 'doom-system-cpus 'cached-value)
+    (if (fboundp 'num-processors)
+        (num-processors) ; added in Emacs 28.1
       (let ((cpus
              (cond ((fboundp 'w32-get-nproc)
                     (w32-get-nproc))
@@ -105,7 +107,7 @@ Tries to be portable. Returns 1 if cannot be determined."
                       (string-to-number (cdr cpus))
                     (user-error "Failed to look up number of processors, because:\n\n%s"
                                 (cdr cpus)))))
-             1)))))
+               1))))))
 
 (provide 'doom-lib '(system))
 ;;; system.el ends here