selftests/bpf: Add DENYLIST.riscv64

This patch adds DENYLIST.riscv64 file for riscv64. It will help BPF CI
and local vmtest to mask failing and unsupported test cases.

We can use the following command to use deny list in local vmtest as
previously mentioned by Manu.

PLATFORM=riscv64 CROSS_COMPILE=riscv64-linux-gnu- vmtest.sh \
    -l ./libbpf-vmtest-rootfs-2024.08.30-noble-riscv64.tar.zst -- \
    ./test_progs -d \
        \"$(cat tools/testing/selftests/bpf/DENYLIST.riscv64 \
            | cut -d'#' -f1 \
            | sed -e 's/^[[:space:]]*//' \
                  -e 's/[[:space:]]*$//' \
            | tr -s '\n' ','\
        )\"
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-9-pulehui@huaweicloud.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/DENYLIST.riscv64

+# riscv64 deny list for BPF CI and local vmtest
+exceptions					# JIT does not support exceptions
+tailcalls/tailcall_bpf2bpf*			# JIT does not support mixing bpf2bpf and tailcalls