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 4 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
manual build:arch #75757
allowed to fail manual
passed build:asan #75753

00:03:21

passed build:bionic #75756

00:08:21

passed build:buster #75755

00:09:03

passed build:stretch #75754

00:08:09

passed build:ubsan #75752

00:03:24