libbpf: Fixed getting wrong return address on arm64 architecture

ARM64 has a separate lr register to store the return address, so here
you only need to read the lr register to get the return address, no need
to dereference it again.

Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1725787433-77262-1-git-send-email-chengshuyi@linux.alibaba.com