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. https://github.com/cilium/ebpf/discussions/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: #1543 (rebased on https://github.com/ti-mo/ebpf/tree/tb/ebpf-variables plus some required hack)

[lmb]

cc @ti-mo PTAL

[ti-mo]

This was merged in https://github.com/cilium/ebpf/pull/1572. bpf2go work continuing in https://github.com/cilium/ebpf/pull/1543.