bpf: add ld64 imm test cases

Add test cases where we combine semi-random imm values, mainly for testing
JITs when they have different encoding options for 64 bit immediates in
order to reduce resulting image size.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>