ldconfig: filter out non-existing directories from default path

This is done only for ld-elf32.so.1, where /lib32 is kept in the path
due to the way STANDARD_LIBRARY_PATH is defined in rtld_paths.h.

This change fixes innocent warning appearing after
99132daf6f70cb0cc969c555d3612547fa3cf1db

Reviewed by: dim, emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42626