bpf, tests: Add test for 32-bit context pointer argument passing

On a 32-bit architecture, the context pointer will occupy the low
half of R1, and the other half will be zero.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210809091829.810076-12-johan.almbladh@anyfinetworks.com