[RFC] pthread_once: Use unified variant instead of custom x86_64/i386

[Roland McGrath]

> +static int
> +__attribute__((noinline))

Space before paren.

> +__pthread_once_slow (pthread_once_t *once_control, void (*init_routine) (void))

This needs a comment explaining why it's separate.  I would have expected
the compiler to generate essentially the same code for the early bailout
case marked with __glibc_likely.

> +  atomic_read_barrier();
[...]
> +    return __pthread_once_slow(once_control, init_routine);

Space before paren.