[BPF] make test case reloc-btf.ll tolerable for old compilers

The test case reloc-btf.ll is generated with an IR containing
spFlags introduced by https://reviews.llvm.org/rL347806

.
In the case of BTF backporting, the old compiler may not
have this patch, so this test will fail during
validation.

This patch removed spFlags from IR in the test case
and used the old way for various flags.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 354409