whereis: include native multiarch path in lib search paths

whereis: include native multiarch path in lib search paths

This includes atleast the native multiarch path in the paths to search.
(Maybe also other multiarch paths should also be searched? But atleast
this is a first step.)

Before this change (on Debian):
$ whereis libc
libc: /usr/share/man/man7/libc.7.gz

After this change:
$ whereis libc
libc: /usr/lib/x86_64-linux-gnu/libc.so /usr/lib/x86_64-linux-gnu/libc.a /usr/share/man/man7/libc.7.gz

Addresses: https://bugs.debian.org/856968
Signed-off-by: Andreas Henriksson <andreas@fatal.se>