commit 531b698c ("libbpf-tools: Enable compilation warnings for
BPF programs") applies CFLAGS to all targets. However, some of the c
flags typically used by distribution are not available to the bpf
target. Add a new BPFCFLAGS macro to take care of that.

Fixes the following compilation error on fedora:

  BPF      bashreadline.bpf.o
clang-13: warning: optimization flag '-ffat-lto-objects' is not supported [-Wignored-optimization-argument]
clang-13: warning: argument unused during compilation: '-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1' [-Wunused-command-line-argument]
clang-13: warning: argument unused during compilation: '-specs=/usr/lib/rpm/redhat/redhat-annobin-cc1' [-Wunused-command-line-argument]
clang-13: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
error: option 'cf-protection=return' cannot be specified on this target
error: option 'cf-protection=branch' cannot be specified on this target
2 errors generated.