The special vxworks rules for the compilation of libgcc had
-I.../gcc/include and not .../gcc/include-fixed, causing build
failure of our arm-vxworks7r2 port because of indirect dependencies
on limits.h.

The omission was just an oversight and this change just adds the
missing -I,

This fixes the aforementioned build failure, has been used in gcc-9
based production compilers for several targets for a year, passed a build
& test sequence for powerpc-vxworks7 with gcc-10 and a sanity check build
with a recent mainline.

Committing to mainline shortly.

Olivier

2020-09-24  Olivier Hainque  <hainque@adacore.com>

libgcc/
        * config/t-vxworks: Add include-fixed to include search
        paths for libgcc on VxWorks.
        * config/t-vxworks7: Likewise.