selftests/bpf: Fix btfgen tests

There turned out to be a few problems with btfgen selftests.

First, core_btfgen tests are failing in BPF CI due to the use of
full-featured bpftool, which has extra dependencies on libbfd, libcap,
etc, which are present in BPF CI's build environment, but those shared
libraries are missing in QEMU image in which test_progs is running.

To fix this problem, use minimal bootstrap version of bpftool instead.
It only depend on libelf and libz, same as libbpf, so doesn't add any
new requirements (and bootstrap bpftool still implementes entire
`bpftool gen` functionality, which is quite convenient).

Second problem is even more interesting. Both core_btfgen and core_reloc
reuse the same set of struct core_reloc_test_case array of test case
definitions. That in itself is not a problem, but btfgen test replaces
test_case->btf_src_file property with the path to temporary file into
which minimized BTF is output by bpftool. This interferes with original
core_reloc tests, depending on order of tests execution (core_btfgen is
run first in sequential mode and skrews up subsequent core_reloc run by
pointing to already deleted temporary file, instead of the original BTF
files) and whether those two runs share the same process (in parallel
mode the chances are high for them to run in two separate processes and
so not interfere with each other).

To prevent this interference, create and use local copy of a test
definition. Mark original array as constant to catch accidental
modifcations. Note that setup_type_id_case_success() and
setup_type_id_case_success() still modify common test_case->output
memory area, but it is ok as each setup function has to re-initialize it
completely anyways. In sequential mode it leads to deterministic and
correct initialization. In parallel mode they will either each have
their own process, or if core_reloc and core_btfgen happen to be run by
the same worker process, they will still do that sequentially within the
worker process. If they are sharded across multiple processes, they
don't really share anything anyways.

Also, rename core_btfgen into core_reloc_btfgen, as it is indeed just
a "flavor" of core_reloc test, not an independent set of tests. So make
it more obvious.

Last problem that needed solving was that location of bpftool differs
between test_progs and test_progs' flavors (e.g., test_progs-no_alu32).
To keep it simple, create a symlink to bpftool both inside
selftests/bpf/ directory and selftests/bpf/<flavor> subdirectory. That
way, from inside core_reloc test, location to bpftool is just "./bpftool".

v2->v3:
  - fix bpftool location relative the test_progs-no_alu32;
v1->v2:
  - fix corruption of core_reloc_test_case.

Fixes: 704c91e5

 ("selftests/bpf: Test "bpftool gen min_core_btf")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yucong Sun <sunyucong@gmail.com>
Link: https://lore.kernel.org/bpf/20220220042720.3336684-1-andrii@kernel.org