Fixes to linux/bpf/btf.zig

[bfredl]

• the meaning of packed structs changed in zig 0.10. adjust accordingly. Use "extern struct" for the cases that directly map to C structs.

• Add new type info kinds, like enum64 and DeclTag

• change the Type enum to use the canonical names from libbpf. This is more predictable when comparing with external BPF documentation (than invented synonyms that need to be guessed)

cc @mattnite