Use LLD for LTO builds on Mac (#4594)

Mac's LLD now supports ThinLTO and is much faster than ld with libLTO.

Author: dschuff

CMakeLists.txt

@@ -167,9 +167,7 @@ if(BYN_ENABLE_LTO)
   if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     message(FATAL_ERROR "ThinLTO is only supported by clang")
   endif()
-  if(NOT APPLE)
-    add_link_flag("-fuse-ld=lld")
-  endif()
+  add_link_flag("-fuse-ld=lld")
   set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=2)
   set(CMAKE_JOB_POOL_LINK link_job_pool)
   add_compile_flag("-flto=thin")