noise from gcc.dg/torture/fp-convert tests

[Eric Christopher]

So, these are xfailed, but still produce quite a bit of noise on both 
x86_64-darwin and x86_64-linux since they fail to produce a working 
executable and then xfail. Should we move them to skip or link only and 
xfail them. With link only they do manage to be quiet in the logs and 
we'll still notice if they start linking correctly and can move them to 
run then.

thoughts? pre-approved? :)
-eric