Use float stack slots for e500 promoted arguments

[John David Anglin]

> Thanks.  Bootstrapped and tested on x86_64-linux; assuming it fixes PA
> bootstrap, OK to install?

Change fixes PA bootstrap and C test results are ok.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)