Adjust LIBGCC2_INCLUDES for VxWorks and augment comment

Adjust LIBGCC2_INCLUDES for VxWorks and augment comment

Investigating the reasons for libgcc build failures in a canadian
context, orthogonally to the recent update of vxcrtstuff, exposed
interesting differences in the way include search paths are managed
between a regular Linux->VxWorks cross build and a canadian setup
building a Windows->VxWorks toolchain in a Linux environment.

This change augments the comment attached to LIBGCC2_INCLUDE in
libgcc/config/t-vxworks to better describe the parameters at play.

It also adjusts the addition of options for gcc/include and
gcc/include-fixed to minimize the actual differences for libgcc
in the two kinds of configurations.

2022-03-06  Olivier Hainque  <hainque@adacore.com>

libgcc/
	* config/t-vxworks (LIBGCC2_INCLUDE): Augment comment.  Move
	-I options for gcc/include and gcc/include-fixed at the end
	and make them -isystem.