selftests/bpf: Fix sockopt selftest failure on powerpc

selftests/bpf: Fix sockopt selftest failure on powerpc

The SO_RCVLOWAT option is defined as 18 in the selftest header,
which matches the generic definition. However, on powerpc,
SO_RCVLOWAT is defined as 16. This discrepancy causes
sol_socket_sockopt() to fail with the default switch case on powerpc.

This commit fixes by defining SO_RCVLOWAT as 16 for powerpc.

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Link: https://lore.kernel.org/bpf/20250311084647.3686544-1-skb99@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>