bpf: samples: exclude asm/sysreg.h for arm64

commit 338d4f49d6f7114a017d294ccf7374df4f998edc
("arm64: kernel: Add support for Privileged Access Never") includes sysreg.h
into futex.h and uaccess.h. But, the inline assembly used by asm/sysreg.h is
incompatible with llvm so it will cause BPF samples build failure for ARM64.
Since sysreg.h is useless for BPF samples, just exclude it from Makefile via
defining __ASM_SYSREG_H.

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/Makefile

@@ -67,10 +67,13 @@ HOSTLOADLIBES_lathist += -lelf
 # point this to your LLVM backend with bpf support
 LLC=$(srctree)/tools/bpf/llvm/bld/Debug+Asserts/bin/llc
 
+# asm/sysreg.h inline assmbly used by it is incompatible with llvm.
+# But, ehere is not easy way to fix it, so just exclude it since it is
+# useless for BPF samples.
 $(obj)/%.o: $(src)/%.c
 	clang $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
-		-D__KERNEL__ -Wno-unused-value -Wno-pointer-sign \
+		-D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
 		-O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf -filetype=obj -o $@
 	clang $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
-		-D__KERNEL__ -Wno-unused-value -Wno-pointer-sign \
+		-D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
 		-O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf -filetype=asm -o $@.s