libbpf: Fix accessing syscall arguments on powerpc

powerpc does not select ARCH_HAS_SYSCALL_WRAPPER, so its syscall
handlers take "unpacked" syscall arguments. Indicate this to libbpf
using PT_REGS_SYSCALL_REGS macro.

Reported-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Link: https://lore.kernel.org/bpf/20220209021745.2215452-5-iii@linux.ibm.com

tools/lib/bpf/bpf_tracing.h

@@ -180,6 +180,8 @@
 #define __PT_RC_REG gpr[3]
 #define __PT_SP_REG sp
 #define __PT_IP_REG nip
+/* powerpc does not select ARCH_HAS_SYSCALL_WRAPPER. */
+#define PT_REGS_SYSCALL_REGS(ctx) ctx
 
 #elif defined(bpf_target_sparc)