amd64: add an i386 include directory

This directory will hold i386-specific headers that are needed for
-m32 support on amd64 and where the amd64 and i386 cases have too
little in common for combining them to make sense. Files to be
installed will come in later commits.

With the currently required set of files, this could be done with
another INCGROUP in include/Makefile, but at least one file that
might want -m32 support (ieeefp.h) conflicts with a files installed
in /usr/include.

Reviewed by: jhb, imp