selftests/bpf: Don't rely on preserving volatile in PT_REGS macros in loop3

PT_REGS*() macro on some architectures force-cast struct pt_regs to
other types (user_pt_regs, etc) and might drop volatile modifiers, if any.
Volatile isn't really required as pt_regs value isn't supposed to change
during the BPF program run, so this is correct behavior.

But progs/loop3.c relies on that volatile modifier to ensure that loop
is preserved. Fix loop3.c by declaring i and sum variables as volatile
instead. It preserves the loop and makes the test pass on all
architectures (including s390x which is currently broken).

Fixes: 3cc31d79

 ("libbpf: Normalize PT_REGS_xxx() macro definitions")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220106205156.955373-1-andrii@kernel.org