samples: bpf: Suppress readelf stderr when probing for BTF support

When compiling bpf samples, the following warning appears:

readelf: Error: Missing knowledge of 32-bit reloc types used in DWARF
sections of machine number 247
readelf: Warning: unable to apply unsupported reloc type 10 to section
.debug_info
readelf: Warning: unable to apply unsupported reloc type 1 to section
.debug_info
readelf: Warning: unable to apply unsupported reloc type 10 to section
.debug_info

Same problem was mentioned in commit 2f09212 ("selftests/bpf:
suppress readelf stderr when probing for BTF support"), let's use
readelf that supports btf.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20211021123913.48833-1-pulehui@huawei.com

samples/bpf/Makefile

@@ -229,6 +229,7 @@ CLANG ?= clang
 OPT ?= opt
 LLVM_DIS ?= llvm-dis
 LLVM_OBJCOPY ?= llvm-objcopy
+LLVM_READELF ?= llvm-readelf
 BTF_PAHOLE ?= pahole
 
 # Detect that we're cross compiling and use the cross compiler
@@ -252,7 +253,7 @@ BTF_PAHOLE_PROBE := $(shell $(BTF_PAHOLE) --help 2>&1 | grep BTF)
 BTF_OBJCOPY_PROBE := $(shell $(LLVM_OBJCOPY) --help 2>&1 | grep -i 'usage.*llvm')
 BTF_LLVM_PROBE := $(shell echo "int main() { return 0; }" | \
 			  $(CLANG) -target bpf -O2 -g -c -x c - -o ./llvm_btf_verify.o; \
-			  readelf -S ./llvm_btf_verify.o | grep BTF; \
+			  $(LLVM_READELF) -S ./llvm_btf_verify.o | grep BTF; \
 			  /bin/rm -f ./llvm_btf_verify.o)
 
 BPF_EXTRA_CFLAGS += -fno-stack-protector