Extract CPU counting to doom-num-cpus function

And improves macOS support as discussed in hlissner/doom-emacs@db6a27c

core/autoload/process.el

@@ -40,3 +40,21 @@ Warning: freezes indefinitely on any stdin prompt."
               (sit-for 0.1))
             (process-exit-status process))
           (string-trim (buffer-string)))))
+
+(defvar doom-num-cpus nil)
+;;;###autoload
+(defun doom-num-cpus ()
+  "Return the max number of processing units on this system.
+Tries to be portable. Returns 1 if cannot be determined."
+  (or doom-num-cpus
+      (setq doom-num-cpus
+            (max
+             1 (cond ((eq 'windows-nt system-type)
+                      (or (ignore-errors
+                            (string-to-number (getenv "NUMBER_OF_PROCESSORS")))
+                          1))
+                     ((executable-find "nproc")
+                      (string-to-number (cdr (doom-call-process "nproc"))))
+                     ((and IS-MAC (executable-find "sysctl"))
+                      (string-to-number (cdr (doom-call-process "sysctl" "-n" "hw.ncpu"))))
+                     (t 1))))))