selftests/bpf: Fix vmlinux test on s390x

Use a syscall macro to access the nanosleep()'s first argument;
currently the code uses gprs[2] instead of orig_gpr2.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-18-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>