[PATCH] Add optional __Bfloat16 support

[Florian Weimer]

* liuhongt via Libc-alpha:

> +\subsubsection{Special Types}
> +
> +The \code{__Bfloat16} type uses a 8-bit exponent and 7-bit mantissa.
> +It is used for \code{BF16} related intrinsics, it cannot be
> +used with standard C operators.

I think it's not necessary to specify whether the type supports certain
C operators (surely assignment will work?).  If they are added later,
the ABI won't need changing.

Thanks,
Florian