include/i386: fix release builds

Add a target to ensure the presence of /usr/include/i386
before we try to install files. In the buildworld/installworld path
it gets created different ways, but for distribute (used in releases)
we need it in the subdir.

(Creating of this the x86, pc, powerpc directories is far too complex.
We should look for ways to simplify it.)

Reported by: lwhsu
Fixes: a09ea2bbc3053d3b2afdb2f7444e5e57a96bb891