selftests/bpf: suppress readelf stderr when probing for BTF support

selftests/bpf: suppress readelf stderr when probing for BTF support

Before:
$ make -s -C tools/testing/selftests/bpf
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

After:
$ make -s -C tools/testing/selftests/bpf

v2:
* use llvm-readelf instead of redirecting binutils' readelf stderr to
  /dev/null

Signed-off-by: Stanislav Fomichev <[email protected]>
Acked-by: Song Liu <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>