Clang is required in GCC docker build images by BPF programs while building kselftests

BPF has a separate suite in kselftests. Often BPF tests are present in other suites like in `net`. BPF programs are built with clang by default. Out GCC docker build images don't have clang for obvious reasons. This can cause failures and pollute the kselftests build results.

```
kernelci@b99ec1587430:~/tools/testing/selftests/net$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j `nproc`
/bin/sh: 1: clang: not found
BPF_PROG nat6to4.bpf.o
make: clang: No such file or directory
make: *** [bpf.mk:42: /home/kernelci/tools/testing/selftests/net/nat6to4.bpf.o] Error 127
```

Possibly this should be fixed in kselftests Makefiles. When compiler is the GCC, it should ignore build of BPF programs.

Start with tools/testing/selftests/net and the reported bpf.mk:42 rule, then inspect the kselftests Makefiles used by GCC Docker build images. Reproduce the failure with the shown make command. Done means the GCC-only image builds the relevant kselftests without requiring clang or producing clang-not-found failures.

Written by the indexing model from the issue text.