selftests/bpf: Extract syscall wrapper

Extract the helper to set up SYS_PREFIX for fentry and kprobe selftests
that use __x86_sys_* attach functions.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Kenta Tada <Kenta.Tada@sony.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220124141622.4378-2-Kenta.Tada@sony.com