elf_reader: add tests for ELF objects without BTF

[puwun]

Adds infrastructure to test loading of ELF objects compiled without BTF information, verifying the fix from #1807

Changes:

• Added testdata/loader_nobtf.c using legacy map definitions.
• Updated Makefile to compile this new test data with -g0 (no BTF).
• Added TestLoadCollectionSpecWithoutBTF to verify successful parsing when spec.Types is nil.

Fixes: #1810

[ti-mo]

@puwun Thanks for picking this up! After looking at this for a bit, I think it would be preferable to reuse the existing loader.c as much as possible to increase coverage. I suggest the following:

• move BTF map definitions to a new loader_maps_btf.h
• create a new loader_maps_bpf.h with equivalent bpf_map_def-based definitions, ignoring {bpf,btf}_decl_map since their purpose is testing decl tags on maps, which doesn't exist pre-BTF
• #ifndef __NOBTF__, #include "loader_maps_btf.h", #else, #include "loader_maps_bpf.h"
• build loader.c twice, additionally specifying -D__NOBTF__ along with the -g0 flag
• Add a subtest to TestLoadCollectionSpec for the no-btf version. You'll want to do a similar thing with the CollectionSpec under test where you populate a part of CollectionSpec.Maps in the BTF-enabled subtest, since some maps will only be present when BTF is enabled.

Try to make only the strictly-necessary changes to keep things straightforward to review. Thank you!

[puwun]

@ti-mo thanks, updated 👍