ldlibs: Try loading Exherbo architecture-specific ld.so cache first

I'm using Exherbo as an example of a host OS with an unusual layout that
breaks libcapsule's assumptions. Tested in an x86_64 Docker container, but
i386 is symmetrical and should also work.  This also puts the framework
in place to deal with other distributions' architecture-specific ld.so
caches.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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