selftests/bpf: Add 6-argument syscall tracing test

Turns out splice() is one of the syscalls that's using current maximum
number of arguments (six). This is perfect for testing, so extend
bpf_syscall_macro selftest to also trace splice() syscall, using
BPF_KSYSCALL() macro. This makes sure all the syscall argument register
definitions are correct.

Suggested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Alan Maguire <alan.maguire@oracle.com> # arm64
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com> # s390x
Link: https://lore.kernel.org/bpf/20230120200914.3008030-25-andrii@kernel.org