[wide-int] Add fast path for hosts with HWI widening multiplication

[Richard Henderson]

On 05/08/2014 01:12 PM, Richard Sandiford wrote:
> 	* wide-int.cc: Only include longlong.h if W_TYPE_SIZE==32 or
> 	__SIZEOF_INT128__ is defined.

FWIW, this looks pretty good to me.


r~