[Cfp-interest] _Float64x conditional requirement

[Jim Thomas]

The example in section 4 of the Functions spec states that an 
implementation supporting _Float128 must support _Float64x (which may be 
_Float128). However, the current Formats spec doesn't require this. 
Changing the Formats spec (to require _Float64x if _Float128 is 
supported) should make code using _Float64x more portable. Your thoughts?

-Jim