[SOL] Debug sections relocations

[jawilk]

This is an expansion of #25

I was discussing with @terorie how we might get complete debug info back in.

After inspecting some objdumps I found ELF::R_BPF_64_ABS32 relocations in .debug_* sections only reference relocatable symbols in other .debug_* sections (mostly .debug_{str, abbrev, frame}). So these can be filtered by checking the absence of the ELF::SHF_ALLOC flag in the target symbol's section header.

ELF::R_BPF_64_ABS64 in .debug_* sections reference symbols in the .text section and I couldn't find a fix within BPFELFObjectWriter::getRelocType() so I had to resort to the ELFObjectWriter.cpp file. I am not sure if this is acceptable since this is a more general ObjectWriter used by other Archs as well.

The tests in #25 failed because in the final shared object .data.rel.ro (and .eh_frame before the recent commit in solana-labs/rust to discard it) also have ELF::R_BPF_64_ABS64 relocations. The fix is identifying all .debug_* sections and emitting ELF::R_BPF_64_ABS64 only for them. This check could be made more explicit with

FixupSection.getName().startswith(".debug_")

instead of

!(Flags & ELF::SHF_ALLOC)

if needed in this line

https://github.com/jawilk/llvm-project/blob/e0ead099cacad6535137b94a5b9fb9880eb4c43a/llvm/lib/MC/ELFObjectWriter.cpp#L1461

After these changes the tests in solana-labs/solana/programs/bpf passed for me and I got complete DWARF info which I could load successfully into gdb.