selftests, bpf: Fix broken riscv build

This patch is closely related to commit 6016df8f

 ("selftests/bpf:
Fix broken riscv build"). When clang includes the system include
directories, but targeting BPF program, __BITS_PER_LONG defaults to
32, unless explicitly set. Work around this problem, by explicitly
setting __BITS_PER_LONG to __riscv_xlen.

Signed-off-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211028161057.520552-5-bjorn@kernel.org