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 master in 8 minutes and 29 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
manual #75767
allowed to fail manual
build:arch
passed #75763
build:asan

00:03:14

passed #75766
build:bionic

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passed #75765
build:buster

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passed #75764
build:stretch

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passed #75762
build:ubsan

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