btf: handle signed enums and fix default signedness

[lmb]

btf: handle signed enums and fix default signedness

Enums now carry signedness encoded in the KindFlag, with the default being
an unsigned enum. This breaks our assumption that enums are signed by default.

Add Enum.Signed and switch EnumValue.Value to uint64. I used uint64 instead of
uint32 since we'll likely need 64 bits to support BTF_KIND_ENUM64 in the future.

Users of bpf2go that generated enums will see their generated types changing from
signed to unsigned types.

Updates #780

btf: reject invalid structs when formatting

GoFormatter currently outputs nonsensical padding given a struct definition
like the following:

    &Struct{
        Size: 1,
        Members: []Member{
             {Name: "foo", Type: &Int{Size: 2}, Offset: 0},
        },
    }

The reason is that the member is large than the enclosing struct. Return
an error in such cases instead of formatting a bogus type.

btf: fix the output of GoFormatter for small enums

When adding support for sized enums I only adjusted the code that
writes out a type declaration, but forgot to change the output
of enum type literals. This would lead to the wrong type being used
for an enum that is used as a struct field.

Fix this by moving the code from writeTypeDecl to writeTypeLit.

[lmb]

Source: https://patchwork.kernel.org/project/netdevbpf/patch/20220607062600.3716578-1-yhs@fb.com/