[PATH][Ada] Fix multilib handling of gnat.dg when RUNTESTFLAGS is used

[Eric Botcazou]

> Oops, sorry for that. I'm testing right now on 4.4 and trunk with the
> additional --GCC (and I'll fix the spurious whitespace diff between
> trunk and 4.4 on gnat.exp).

That doesn't seem to easily work with gnatlink.  Btw, any particular reason 
for adding -B$root after -margs too?

> Out of curiosity, under what condition does this issue show up?

32-bit compiler on a 64-bit system, e.g. i586-suse-linux on x86_64-suse-linux.

-- 
Eric Botcazou