Enable direct methods and fast alloc calls for libobjc2. (#78030)

These will be supported in the upcoming 2.2 release and so are gated on
that version.

Direct methods call `objc_send_initialize` if they are class methods
that may not have called initialize. This is guarded by checking for the
class flag bit that is set on initialisation in the class. This bit now
forms part of the ABI, but it's been stable for 30+ years so that's fine
as a contract going forwards.