prelink: Fix segfaults in prelink-rtld

prelink-rtld was segfaulting because prelink looks for
libs in lib64 folder for 64-bit machines, however on riscv
the directories are simply /lib and /usr/lib

Secondly define the ldso name for riscv

backport another patch to fix DT_NEEDED processing

Signed-off-by: Khem Raj <raj.khem@gmail.com>