libbpf: Add missing per-arch include path

libbpf does 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 libbpf 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/20240927131355.350918-1-bjorn@kernel.org