Make pthreadpool not use GCD on apple platforms (#2839)

Summary:
Pull Request resolved: #2839

My local M1 runs reveal that GCG was not great as a threadpool

Reviewed By: digantdesai, iseeyuan

Differential Revision: D55712512

fbshipit-source-id: 10e7bc6782dbfcfe0b8cea3cb801fe4ef6706b9d

Author: kimishpatel

CMakeLists.txt

@@ -224,6 +224,11 @@ if(EXECUTORCH_BUILD_PTHREADPOOL)
   set(PTHREADPOOL_ALLOW_DEPRECATED_API
       ON
       CACHE BOOL "")
+  if(APPLE)
+    set(PTHREADPOOL_SYNC_PRIMITIVE
+       "condvar"
+       CACHE STRING "")
+  endif()
   add_subdirectory("${PTHREADPOOL_SOURCE_DIR}")
 endif()