selftests/bpf: Add arm target register definitions

eBPF "restricted C" code can be compiled with LLVM/clang using target
triplets like armv7l-unknown-linux-gnueabihf and loaded/run with small
cross-compiled gobpf/elf [1] programs without requiring a full BCC
port which is also undesirable on small embedded systems due to its
size footprint. The only missing pieces are these helper macros which
otherwise have to be redefined by each eBPF arm program.

[1] https://github.com/iovisor/gobpf/tree/master/elf



Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>