Implement C _FloatN, _FloatNx types [version 5]

[Joseph Myers]

On Wed, 17 Aug 2016, James Greenhalgh wrote:

> The obvious fix would be this modification to fp-int-convert.h:
> 
> -  TEST_I_F_VAL (U, F, (U)~(((U)~(U)0) >> 1), 1);               \
> +  TEST_I_F_VAL (U, F, (U)~(((U)~(U)0) >> 1), P_OK1 (P, U));    \

That's not logically correct, because this is an issue about exponent 
range, not precision; it's fine to test this input value when converting 
from TImode to float, for example, because it's 2**127 which is exactly 
representable in float.

I'll do another revision of the patch that addresses this and has other 
testsuite improvements (but no changes to the actual compiler changes, 
which still need review for language-independent changes outside the 
rs6000 back end).

-- 
Joseph S. Myers
joseph@codesourcery.com