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

[musamaanjum]

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.