[i386] Fix couple of issues in large PIC model on x86-64/VxWorks

[Eric Botcazou]

> LGTM for the generic part, no idea for VxWorks.

Thanks.  The VxWorks-specific hunk is needed to make GCC compatible with the 
system compiler on this architecture (LLVM) and I have CCed Olivier.

-- 
Eric Botcazou