xdp-tools: fix compilation wrongly using host header

xdp-tools: fix compilation wrongly using host header

Currently it's needed to have gcc-multilib on the host to correctly
compile xdp-tools. This is wrong and means that we are using host header
to compile a tool.

By some searching in how the makefile works it was discovered that
BPF_CFLAGS were not used and required to be appended to config.mk

Only one single header was added but we should include each BPF_CFLAGS
from bpf.mk. To make this some patching to bpf-header were required and
some patches to xdp-tools were required.
Also it's needed to pass the correct target to BPF_CFLAGS.

With the following changes xdp-tools can correctly compile with each
header from bpf-headers and should not use any host header.

Co-Developed-by: Andre Heider <a.heider@gmail.com>
Signed-off-by: Andre Heider <a.heider@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/11825
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>