including arch includes per MACHINE_ARCH not per MACHINE

[David Brownlee <abs%absd.org@localhost>]

        Each port currently ships its own arch includes in
        /usr/include/$arch with a /usr/include/machine symlink.

        I'd like to propose we include the arch includes for all
        machines of a given MACHINE_ARCH

        So, the comp set for arm machines would include:

        /usr/include/
        {acorn26,acorn32,cats,evbarm,hpcarm,iyonix,netwinder,shark,zaurus}

        This is potentially a step closer to making userland sharable
        per machine_arch, and makes it possible to build more arch
        specific packages on machines of the same machine_arch (which
        bit me on pkgsrc recently), including potentially X servers.

        Ideally of course most of the includes from the above *should*
        be in the already shared /usr/include/arm anyway.

        Note, /usr/include/machine will remain the same symlink, and
        as an added bonus will not add any files to the vax build
        (which is its own machine_arch).