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...

headers/build/os/kernel/fs_attr.h

@@ -23,6 +23,7 @@ extern "C" {
 
 /* Since libroot_build is also used on Haiku and linked against the real
  * libroot which also has the fs*attr functions, these must be shadowed. */
+#ifndef BUILDING_FS_ATTR_HAIKU
 #define fs_read_attr		build_fs_read_attr
 #define fs_write_attr		build_fs_write_attr
 #define fs_remove_attr		build_fs_remove_attr
@@ -35,6 +36,7 @@ extern "C" {
 #define fs_close_attr_dir	build_fs_close_attr_dir
 #define fs_read_attr_dir	build_fs_read_attr_dir
 #define fs_rewind_attr_dir	build_fs_rewind_attr_dir
+#endif
 
 
 extern ssize_t	fs_read_attr(int fd, const char *attribute, uint32 type,