libbpf: Fix accessing syscall arguments on riscv

riscv 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: NHeiko Carstens <hca@linux.ibm.com>
Signed-off-by: NIlya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220209021745.2215452-7-iii@linux.ibm.com

tools/lib/bpf/bpf_tracing.h

@@ -214,6 +214,8 @@
 #define __PT_RC_REG a5
 #define __PT_SP_REG sp
 #define __PT_IP_REG pc
+/* riscv does not select ARCH_HAS_SYSCALL_WRAPPER. */
+#define PT_REGS_SYSCALL_REGS(ctx) ctx
 
 #endif