samples/bpf: Fix compilation errors with cf-protection option

Currently, compiling the bpf programs will result the compilation errors
with the cf-protection option as follows in arm64 and loongarch64 machine
when using gcc 12.3.1 and clang 17.0.6. This commit fixes the compilation
errors by limited the cf-protection option only used in x86 platform.

[root@localhost linux]# make M=samples/bpf
	......
  CLANG-bpf  samples/bpf/xdp2skb_meta_kern.o
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.
  CLANG-bpf  samples/bpf/syscall_tp_kern.o
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.
	......

Fixes: 34f6e38f ("samples/bpf: fix warning with ignored-attributes")
Reported-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Qiang Wang <wangqiang1@kylinos.cn>
Link: https://lore.kernel.org/bpf/20240815135524.140675-1-13667453960@163.com

samples/bpf/Makefile

@@ -169,6 +169,10 @@ BPF_EXTRA_CFLAGS += -I$(srctree)/arch/mips/include/asm/mach-generic
 endif
 endif
 
+ifeq ($(ARCH), x86)
+BPF_EXTRA_CFLAGS += -fcf-protection
+endif
+
 TPROGS_CFLAGS += -Wall -O2
 TPROGS_CFLAGS += -Wmissing-prototypes
 TPROGS_CFLAGS += -Wstrict-prototypes
@@ -405,7 +409,7 @@ $(obj)/%.o: $(src)/%.c
 		-Wno-gnu-variable-sized-type-not-at-end \
 		-Wno-address-of-packed-member -Wno-tautological-compare \
 		-Wno-unknown-warning-option $(CLANG_ARCH_ARGS) \
-		-fno-asynchronous-unwind-tables -fcf-protection \
+		-fno-asynchronous-unwind-tables \
 		-I$(srctree)/samples/bpf/ -include asm_goto_workaround.h \
 		-O2 -emit-llvm -Xclang -disable-llvm-passes -c $< -o - | \
 		$(OPT) -O2 -mtriple=bpf-pc-linux | $(LLVM_DIS) | \