selftests/bpf: Enable cross-building

Build bpftool and resolve_btfids using the host toolchain when
cross-compiling, since they are executed during build to generate the
selftests. Add a host build directory in order to build both host and
target version of libbpf. Build host tools using $(HOSTCC) defined in
Makefile.include.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113163319.1516382-2-jean-philippe@linaro.org