dwarf_loader: Support for btf:type_tag

"btf:type_tag" is an DW_TAG_LLVM_annotation object that encodes
btf_type_tag attributes in DWARF. Contrary to existing "btf_type_tag"
it allows to associate such attributes with non-pointer types.
When "btf:type_tag" is attached to a type it applies to this type.

For example the following C code:

    struct echo {
      int __attribute__((btf_type_tag("__c"))) c;
    }

Produces the following DWARF:

0x29:   DW_TAG_structure_type
          DW_AT_name      ("echo")

0x40:     DW_TAG_member
            DW_AT_name    ("c")
            DW_AT_type    (0x8c "int")

0x8c:   DW_TAG_base_type
          DW_AT_name      ("int")
          DW_AT_encoding  (DW_ATE_signed)
          DW_AT_byte_size (0x04)

0x90:     DW_TAG_LLVM_annotation
            DW_AT_name    ("btf:type_tag")
            DW_AT_const_value     ("__c")

Meaning that type 0x8c is an `int` with type tag `__c`.
Corresponding BTF looks as follows:

[1] STRUCT 'echo'
        ...
        'c' type_id=8 bits_offset=128
[4] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[8] TYPE_TAG '__c' type_id=4

This commit adds support for DW_TAG_LLVM_annotation "btf:type_tag"
attached to the following entities:
- base types;
- arrays;
- pointers;
- structs
- unions;
- enums;
- typedefs.

To allow backwards compatibility and void additional invocation
options, implementation acts in a following way:
- both `btf_type_tag` and `btf:type_tag` could be present in the
  debug info;
- if `btf:type_tag` are present in the debug info, `btf_type_tag`
  annotations are ignored.

Modifications done by this commit:
- DWARF load phase is updated:
  - `annots` fields are filled for the above mentioned types;
  - `cu` instance is updated to reflect which version of type tags is
    used in the debug info;
- Recode phase is split in several sub-phases:
  - `cu__allocate_btf_type_tags()`
    `llvm_annotation` instances corresponding to preferred version of
    type tags are added to types table;
  - `tag__recode_dwarf_type()` (the original phase logic);
  - `update_btf_type_tag_refs()`
    Updates `->type` field of each tag if that type refers to a type
    with `btf:type_tag` annotation. The id of the type is replaced by
    id of the type tag.

See also:
[1] Mailing list discussion regarding `btf:type_tag`
    Various approaches are discussed, Solution acmel#2 is accepted
    https://lore.kernel.org/bpf/87r0w9jjoq.fsf@oracle.com/

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>