selftests/bpf: Add fexit_sleep to DENYLIST.aarch64

It is reported that the fexit_sleep never returns in aarch64.
The remaining tests cannot start. Put this test into DENYLIST.aarch64
for now so that other tests can continue to run in the CI.

Acked-by: Manu Bretelle <chantr4@gmail.com>
Reported-by: Manu Bretelle <chantra@meta.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>