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>
6 jobs for wip/exherbo in 9 minutes and 37 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
manual #63440
allowed to fail manual
build:arch
passed #63436
build:asan

00:03:47

passed #63439
build:bionic

00:09:36

passed #63438
build:buster

00:07:43

passed #63437
build:stretch

00:08:08

passed #63435
build:ubsan

00:03:14