Before this commit, the headers in plat/osx/include got installed
twice into PLATIND/osx386/include and PLATIND/osxppc/include. This
commit installs them once into PLATIND/osx/include and changes both
descr files to find them.
Several rules in lang/ depend on plat/osx386/include+headers or
plat/osxppc/include+headers. They each become a simplerule that
depends on plat/osx/include+headers.
