selftests/bpf: Define SYS_PREFIX for powerpc

Since commit 7e92e01b ("powerpc: Provide syscall wrapper")
landed in v6.1, syscall wrapper is enabled on powerpc. Commit
94746890 ("powerpc: Don't add __powerpc_ prefix to syscall
entry points") , that drops the prefix to syscall entry points,
also landed in the same release. So, add the missing empty
SYS_PREFIX prefix definition for powerpc, to fix some fentry
and kprobe selftests.

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/7192d6aa9501115dc242435970df82b3d190f257.1738302337.git.skb99@linux.ibm.com