glibc_multi: fix ldd on x86_64

This patch will fix ldd on x86_64 multi arch systems.

The problem:
ldd will check a given binary against every rtld in a list via $rtld --verify.
Usually this list contains both a path of the 32bit and the 64bit linker.

The problem on NixOS is that these libraries don't correspondent in the same directory since they belong to two different packages--but ldd still thinks so.

That causes that ldd from glibc_multi will only work on 64bit binaries and will exit with "not a dynamic linked library" otherwise.

This fix simply corrects the RTLDLISD variable used in ldd.