Compile-time switch for custom CO-RE BPF

[erthalion]

Implement compile-time option to turn the custom CO-RE BPF collection method
on/off (by off we mean using vanilla CO-RE BPF). Implementing this at runtime
requires too invasive changes into libpman, thus do whatever is easier:

• Embed two collector binaries with vanilla & custom CO-RE BPF probe skeleton.
  This has to be very explicit about which one is in use, because otherwise it
  would be easy to use a wrong method unintentionally. So, it has to be logged,
  and for CI purposes even enforced from the outside (so that if something is
  mixed up the result would be a failure, not silently incorrect tests).

• Produce two images with different suffix. It's probably less appealing, since
  we try to go away from having many flavours of the same image, but still a
  feasible option. The same as above applies, it has to be clear which one is
  tested on CI.

Part of #1320