gas: implement parsing of object attributes v2

This patch adds the parsing logic for Object Attributes v2 (OAv2), enabling
Gas to interpret and process these attributes correctly. It also updates the
AArch64 backend to utilize the new parsing capabilities, and handle the new
AArch64-specific directives.

This patch relies on the abstractions introduced in the previous patch to
store the data. Its scope is limited to parsing the new assembly directives,
checking the inputs, and storing the data into the relevant OAv2 abstractions.
Note that, for now, the new parsing capabilities are only available for AArch64.
Even if the implementation was splitted into a generic part available in
gas/config/obj-elf.c, and an AArch64-specific one in gas/config/tc-aarch64.c,
the lack of GNU generic directives to handle OAv2 prevented the capability
from being exposed to others backends.

** GNU assembler interface for aeabi subsections

OAv2 introduced two new directives for AArch64:
- .aeabi_subsection name, comprehension, encoding
  Create or switch the current subsection to 'name'.
  Comprehension values can be 'required' or 'optional'.
  Encoding values are limited for now to 'ULEB128', and 'NTBS'
  The comprehension and encoding are mandatory arguments on the first
  declaration of the subsection, but become optional for subsequent
  ones.
- .aeabi_attribute tag, value
  Set 'tag' to 'value' in the current subsection.
  Tag can either be an integer, or one of the defined symbols in the backend.

The usage of those directives will error if the following requirements
are breached:
- If the subsection X has been previously declared, the comprehension and
  encoding parameters of the current .aeabi_subsection that redeclares X
  have to match with the previous declaration. If those parameters are
  omited, no check is performed.
- The type of the value set via .aeabi_attribute has to align with
  the current subsection.
- If the tag N has already been declared for the current subsection, a
  later assignment to tag N is tolerated only if the newly set value is
  equal to the former one. This check is stricter than needed. Ideally,
  the tag N's values should be merged together, and an error should be
  raised only if an incompatibility is detected. Because the attributes
  are set in one chunk by GCC, there is no real use case for such a merge.

The new parsing code is enabled/disabled via the TC_OBJ_ATTR_v1 and
TC_OBJ_ATTR_v2 defines, and supports the following configurations:
- enable both OAv1 and OAv2 parsing. This is currently used by no
  target, but is useful for migration from OAv1 to OAv2.
- enable OAv1 parsing only. This is used by all targets supporting OAs
  except for AArch64.
- enable OAv2 parsing only. This is only used by AArch64.

** Regarding the implementation

The logic of OAv1 does not always keep separated the different data processing
steps: parsing, convertion to internal abstractions, error checking and further
processing (if any) on those abstractions, and their serialization into the
object files.
This patch takes into account the specifities of syntax for OAv1 and OAv2, but
mutualize as much as possible the common behavior so that the same methods can
be used for parsing the OAv1 and OAv2 directives.
However, the mutualization of the code is limited by a different internal model
for OAv1 and OAv2. Even if it is technically feaseable to have only one middle
-end for OAv1, OAv2 and even GNU properties, sharing the same data model to
perform the merge logic with the same code, it is a significant amount of work.
This extra work was not considered as a part of this new feature, so this patch
series will stick with the minimum of mutualization as possible.

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