bpf/tests: Add more BPF_END byte order conversion tests

This patch adds tests of the high 32 bits of 64-bit BPF_END conversions.

It also adds a mirrored set of tests where the source bytes are reversed.
The MSB of each byte is now set on the high word instead, possibly
affecting sign-extension during conversion in a different way. Mainly
for JIT testing.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210914091842.4186267-13-johan.almbladh@anyfinetworks.com