selftests/bpf: fix bpf_target_sparc check

bpf_helpers.h fails to compile on sparc: the code should be checking
for defined(bpf_target_sparc), but checks simply for bpf_target_sparc.

Also change #ifdef bpf_target_powerpc to #if defined() for consistency.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>