selftests/bpf: Mark tests that require unaligned memory access

A lot of tests require unaligned memory access to work. Mark the tests
as such, so that they can be avoided on unsupported architectures such
as RISC-V.

Signed-off-by: Björn Töpel <bjorn.topel@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Luke Nelson <luke.r.nels@gmail.com>
Link: https://lore.kernel.org/bpf/20201118071640.83773-4-bjorn.topel@gmail.com