CI: Cut Hermes compile times in half on macOS jobs (facebook#713)

Summary:
Pull Request resolved: facebook#713

The `macos.x86.medium.gen2` machine type should cut Hermes compile times in half on macOS, and achieve similar results on all jobs that use the macOS executor.

Reviewed By: neildhar

Differential Revision: D35202784

fbshipit-source-id: b3a88f3a5ba6ad9c9dc543e7d8352ba802afdb9f

.circleci/config.yml

@@ -46,6 +46,7 @@ workflows:
 apple_defaults: &apple_defaults
   macos:
     xcode: 12.4.0
+  resource_class: macos.x86.medium.gen2
   working_directory: ~/hermes
   environment:
     - TERM: dumb