selftests/bpf: Add description for running vmtest on RV64

Add description in tools/testing/selftests/bpf/README.rst
for running vmtest on RV64.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-11-pulehui@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>