Fixes for arm64

[javierhonduco]

On my dev machine:

$ uname -m
arm64

This PR addresses this and creates a GO Make variable to make it easy to
override while trying golang binaries in non-standard paths

Test plan

$ sudo make GO=/usr/local/go/bin/go run-dynamic
[...]
sudo ./run.sh main-dynamic
[*] SUCCESS: all good

cc/ @Sylfrena @v-thakkar @kakkoyun

[kakkoyun]

LGTM 🚀

[grantseltzer]

I never noticed that we were missing this, strange that it's only in those 3 selftests. In the other selftests we have ARCH defined but it's used for GOARCH, not for the clang bpf object compilation. Are the other selftests building for you on your arm machine? For example, the tc selftest?

This is definitely a very welcome fix, and I want to make sure it's consistent across every selftest.

[javierhonduco]

Seems to work! :D

$ [tc] sudo make GO=/usr/local/go/bin/go run-dynamic
make -C /home/vagrant/work/libbpfgo libbpfgo-dynamic
make[1]: Entering directory '/home/vagrant/work/libbpfgo'
CC=clang \
	CGO_CFLAGS= \
	CGO_LDFLAGS="-lelf -lz -lbpf" \
	/usr/local/go/bin/go build .
make[1]: Leaving directory '/home/vagrant/work/libbpfgo'
CC=clang \
	CGO_CFLAGS= \
	CGO_LDFLAGS="-lelf -lz -lbpf" \
	/usr/local/go/bin/go build -o ./main-dynamic ./main.go
sudo ./run.sh main-dynamic
libbpf: Kernel error message: Exclusivity flag on, cannot modify
[*] SUCCESS: all good

https://github.com/aquasecurity/libbpfgo/blob/main/selftest/tc/Makefile is a symlink to ../common/Makefile so I think that's why it works as we are modifying it in this PR 😄

[javierhonduco]

Removed a leftover arch=x86 variable

[grantseltzer]

LGTM!