btf: support CO-RE relocation of ISAv4 instructions

[lmb]

#1193 added support for new instructions. We need to extend COREFixup.Apply to recognise the new load / store modes.

Tasks

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1. Check what libbpf does. Did the upstream patch sets make any changes to CO-RE logic?
2. Implement the same in COREFixup.Apply

[dylandreimerink]

Check what libbpf does. Did the upstream patch sets make any changes to CO-RE logic?

As far as I can find the only change to libbpf for ISAv4 is libbpf/libbpf@3d0e1c5

No changes to the core logic around that same time https://github.com/libbpf/libbpf/commits/master/src/relo_core.c

And the BPF_MEMSX constant isn't used libbpf anywhere, so it seems like no action is required.

[lmb]

We'll need to adjust at least

ebpf/btf/core.go

Line 51 in 36f6316

case asm.LdXClass, asm.StClass, asm.StXClass:

to allow the new class.