Linux: Simplify and fix the definition of SINGLE_THREAD_P

Linux: Simplify and fix the definition of SINGLE_THREAD_P

Always use __libc_multiple_threads if beneficial, and do not assume
the the dynamic loader is single-threaded.  This assumption could
become incorrect by accident once more code is moved from libpthread
into it.  The previous commit introducing the
NO_SYSCALL_CANCEL_CHECKING macro enables this change.

Do not hint to the compiler that multi-threaded programs are unlikely
(which is not quite true anymore).

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>