hppa/ia64: _dl_unmap: make it hidden

This symbol is only used by DL_UNMAP which in turn is only used by
_dl_close_worker in dl-close.c, and _dl_close_worker itself is marked
hidden as it is only used by the ldso.  That means _dl_unmap should
be marked hidden.  Without this, the elf/check-localplt test fails.