libbpf: Provide barrier() and barrier_var() in bpf_helpers.h

Add barrier() and barrier_var() macros into bpf_helpers.h to be used by
end users. While a bit advanced and specialized instruments, they are
sometimes indispensable. Instead of requiring each user to figure out
exact asm volatile incantations for themselves, provide them from
bpf_helpers.h.

Also remove conflicting definitions from selftests. Some tests rely on
barrier_var() definition being nothing, those will still work as libbpf
does the #ifndef/#endif guarding for barrier() and barrier_var(),
allowing users to redefine them, if necessary.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-8-andrii@kernel.org