libroot_build: Full passthrough to system attributes.

As it turns out, using the xattr emulation layer plus "libgnu"
causes some strange mixups at package build time, and so packages
built with it were winding up with no attributes at all.

So I've just bitten the bullet and written a full passthrough layer
to the system attributes. Verified using a full build of haiku.hpkg
this time ... after a lot of painful debugging of symlink mixups.

Hopefully I am finally rid of this plague...