RFC: hex constants in const_int (plus small altivec problem)

[Aldy Hernandez]

> I was the one who filed both reports knowing that the approach for 
> -2147483648
> will be generating an extra warning messages in the code.  A way to 
> have hex in the
> .md files will make sure there is way for no warnings to happen.

Actually, I don't get any warnings on my end, but I still think it is
a good idea.  I did a grep in all the .md files in config/*/ and there
are quite a number of odd looking constants that could benefit from a
hexadecimal representation.

Aldy