Add include-fixed to include search paths for libgcc on VxWorks

Add include-fixed to include search paths for libgcc on VxWorks

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.

2020-10-14  Olivier Hainque  <hainque@adacore.com>

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