selftests/bpf: Asm tests for the verifier regalloc tracking.

Add asm tests for register allocator tracking logic.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201009011240.48506-5-alexei.starovoitov@gmail.com