selftests/bpf: Test BPF_KPROBE_SYSCALL macro

Add tests for the newly added BPF_KPROBE_SYSCALL macro.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220207143134.2977852-3-hengqi.chen@gmail.com