selftests: bpf: Add missing per-arch include path

The prog_tests programs do not include the per-arch tools include
path, e.g. tools/arch/riscv/include. Some architectures depend those
files to build properly.

Include tools/arch/$(SUBARCH)/include in the selftests bpf build.

Fixes: 6d74d178 ("tools: Add riscv barrier implementation")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240927131355.350918-2-bjorn@kernel.org