selftests/bpf: Enable lld usage for RISC-V

RISC-V has proper lld support. Use that, similar to what x86 does, for
urandom_read et al.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231004122721.54525-3-bjorn@kernel.org