libbpf-tools: Allow to use different cflags for bpf targets

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.

libbpf-tools/Makefile

@@ -7,6 +7,7 @@ LIBBPF_SRC := $(abspath ../src/cc/libbpf/src)
 LIBBPF_OBJ := $(abspath $(OUTPUT)/libbpf.a)
 INCLUDES := -I$(OUTPUT) -I../src/cc/libbpf/include/uapi
 CFLAGS := -g -O2 -Wall
+BPFCFLAGS := -g -O2 -Wall
 INSTALL ?= install
 prefix ?= /usr/local
 ARCH := $(shell uname -m | sed 's/x86_64/x86/' | sed 's/aarch64/arm64/' | sed 's/ppc64le/powerpc/' | sed 's/mips.*/mips/')
@@ -107,7 +108,7 @@ $(OUTPUT)/%.skel.h: $(OUTPUT)/%.bpf.o | $(OUTPUT)
 
 $(OUTPUT)/%.bpf.o: %.bpf.c $(LIBBPF_OBJ) $(wildcard %.h) $(ARCH)/vmlinux.h | $(OUTPUT)
 	$(call msg,BPF,$@)
-	$(Q)$(CLANG) $(CFLAGS) -target bpf -D__TARGET_ARCH_$(ARCH)	      \
+	$(Q)$(CLANG) $(BPFCFLAGS) -target bpf -D__TARGET_ARCH_$(ARCH)	      \
 		     -I$(ARCH)/ $(INCLUDES) -c $(filter %.c,$^) -o $@ &&      \
 	$(LLVM_STRIP) -g $@