Summary/motivation of changes:
- FreeBSD has `sys/auxv.h` but it does not have `getauxval`, and its
  equivalent does not have `AT_PLATFORM`. Luckily, we don't actually
  need this at all on FreeBSD since, unlike Linux, libraries aren't put
  into platform-prefixed directories.
- `bash` is not installed by default and when it is, it does not live in
  `/bin`. Thus the integration tests shebang has been changed to use
  `/usr/bin/env bash`, which should work across platforms.
- BSD `diff`, which macOS also uses, does not support the `--color`
  flag, so that now enabled only on Linux.
- Rather than including versioned shared libraries in the exclusion
  list, we can list unversioned library names and strip any version
  number from the library currently being inspected. This accounts for
  cases like FreeBSD's `libc.so.7`, which would have to be added as
  another special case alongside Linux's `libc.so.6` otherwise.

With this change, we use `getauxval` to get the platform on Linux when
that function is available, and when it isn't, we use the POSIX `uname`
to retrieve that info. The `utsname` struct gets passed around as well
so that substitutions other than `$PLATFORM` can be applied downstream
with only one additional function parameter.