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>

Author: Saket Kumar Bhaskar

tools/testing/selftests/bpf/progs/bpf_tracing_net.h

@@ -15,7 +15,11 @@
 #define SO_KEEPALIVE		9
 #define SO_PRIORITY		12
 #define SO_REUSEPORT		15
+#if defined(__TARGET_ARCH_powerpc)
+#define SO_RCVLOWAT		16
+#else
 #define SO_RCVLOWAT		18
+#endif
 #define SO_BINDTODEVICE		25
 #define SO_MARK			36
 #define SO_MAX_PACING_RATE	47