bpftool: add host build

bpftool: add host build

Besides probing BPF information in running system, bpftool is also used in
generating skeleton, dumping BTF, etc. that is widely used in modern BPF
development. Make it available as a host tool so that we can use it in
package build.

Tested build targeting malta/le on Arch Linux x86_64. bpftools currently
does not support processing cross-endian BPF objects, so big-endian host
is needed to build for big-endian targets using bpftools.

Signed-off-by: Eric Long <i@hack3r.moe>
Link: https://github.com/openwrt/openwrt/pull/16122
Signed-off-by: Robert Marko <robimarko@gmail.com>