Serialization and dumping tests of Object Attributes v2

Serialization and dumping tests of Object Attributes v2

The added tests cover the parsing of the new assembly directives
(aeabi_subsection and aeabi_attribute), the serialization of the
Object Attributes v2 (OAv2) data into an object file, and the
dumping of those data via readelf.

The parsing tests focus on the following points:
- the syntax of the new directives.
- the recognition of generic tokens like: NTBS, ULEB128, required,
  optional.
- the recognition of backend-specific tokens like Tag_Feature_(BTI,
  PAC,GCS).

The dumping tests focus on:
- the OAv2 population into the correct section assigned by the backend
  (i.e. .ARM.attributes for AArch64).
- the merge of the subsections and attributes when they are declared
  several times inside respectively the same compilation unit, and
  subsection.
- the sorting of OAv2 before the serialization.

Co-Authored-By: Matthieu Longo <matthieu.longo@arm.com>