New API in Collection{Spec} for modifying global BPF variables

[smagnani96]

Describe the bug

-- Newbie here, text to be adjusted --

Opening this issue to discuss a possible enhancement to the library regarding the interaction with static/global variables in an eBPF program. AFAIU, the current state of the library doesn't allow to interact with such data, since the map containing these varaibles (".rodata", ".data", ".bss") are not emitted (referring to https://github.com/cilium/ebpf/blob/main/cmd/bpf2go/main.go#L367 and https://github.com/cilium/ebpf/blob/main/cmd/bpf2go/gen/output.go#L108).

Useful pointers:

1. how to read global variables directly in a user-mode program #943
2. https://stackoverflow.com/questions/78742654/how-does-mmaped-ebpf-map-shared-between-processes-synchronizes-operations

Ongoing effort:

1. Map.Memory and ebpf.Variable support: https://github.com/ti-mo/ebpf/tree/tb/ebpf-variables
2. bpf2go support for ebpf.Variable: add bpf2go support and wrappers for ebpf.Variable generation #1543 (rebased on https://github.com/ti-mo/ebpf/tree/tb/ebpf-variables plus some required hack)

[lmb]

cc @ti-mo PTAL