selftests/bpf: Fix alignment of .BTF_ids

Fix a build failure on arm64, due to missing alignment information for
the .BTF_ids section:

resolve_btfids.test.o: in function `test_resolve_btfids':
tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140:(.text+0x29c): relocation truncated to fit: R_AARCH64_LDST32_ABS_LO12_NC against `.BTF_ids'
ld: tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140: warning: one possible cause of this error is that the symbol is being referenced in the indicated code as if it had a larger alignment than was declared where it was defined

In vmlinux, the .BTF_ids section is aligned to 4 bytes by vmlinux.lds.h.
In test_progs however, .BTF_ids doesn't have alignment constraints. The
arm64 linker expects the btf_id_set.cnt symbol, a u32, to be naturally
aligned but finds it misaligned and cannot apply the relocation. Enforce
alignment of .BTF_ids to 4 bytes.

Fixes: cd04b04d

 ("selftests/bpf: Add set test to resolve_btfids")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/bpf/20200930093559.2120126-1-jean-philippe@linaro.org