Dispatch macOS 11.0 Beta 2 to Beta 3 API Differences

workgroup_object.h

Removed #def OS_WORKGROUP_ATTR_INITIALIZER

Removed os_workgroup_index

Removed os_workgroup_join_self()

Removed os_workgroup_leave_self()

Removed os_workgroup_set_context()

Removed os_workgroup_get_context()

Added #def OS_WORKGROUP_ATTR_INITIALIZER_DEFAULT

Added os_workgroup_join()

Added os_workgroup_leave()

workloop.h

Added dispatch_workloop_set_os_workgroup()