btf: minimal support for BTF_KIND_DECL_TAG and BTF_KIND_TYPE_TAG

[lmb]

btf: add limited support for BTF_KIND_DECL_TAG and BTF_KIND_TYPE_TAG

Parse decl and type tags from BTF so that we can read BTF on new kernels.
The new types are not exported since the encoding of declTag.Index especially
is cumbersome to use. We can export the types later if necessary.

Treat typeTag as a qualifier, since accoriding to the BTF documentation it
is usually used in a chain of types:

    ptr -> [type_tag]*
        -> [const | volatile | restrict | typedef]*
        -> base_type

Updates #713

tooldeps: add stringer dependency

Pin stringer into go.mod and adjust go:generate directives

[ti-mo]

I think we should look at something like bingo instead of internal/tooldeps. (the package approach is mentioned specifically) Could we drop that change from the PR and address it separately?

[lmb]

Can you tell me why tooldeps doesn't work? What problem does bingo solve that is important to us? On 29 July 2022 16:04:48 Timo Beckers @.***> wrote:

I think we should look at something like bingo instead of internal/tooldeps. Could we drop that change from the PR and address it separately?

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[lmb]

I'm not convinced bingo is the answer, but I dropped the tooldeps from this PR. It's not super urgent anyways.