Introduce a new specification document for AArch64 build attributes.

The 32-bit ABI makes extensive use of the build attributes
defined in addenda32. These attributes defined for relocatable
objects are used for a number of purposes including:

* Checking ABI compatibility.

* Checking required processor feature compatibility.

* Automatic static library selection.

With AArch64's focus on software platforms, there has not been
a strong requirement to introduce extra metadata to denote the
requirements and optional processor features used by a
relocatable object. Programs are expected to do runtime testing
to check if processor features are available.

With the advent of security features like PAuthABI, BTI and GCS
we need a way to record additional metadata that a static linker
can use to set metadata like GNU properties in a loadable-unit
(executable or shared-library).

A brief summary:

* Made a generic "aeabi" prefix for Arm subsections. Including
  a header that states optionality and encoding. All ULEB or
  all NTBS.

* Describes a single subsection that maps directly to the existing
  .note.gnu.properties and uses language similar to AAELF64. This
  includes the existing BTI and PAC properties.

* Is accompanied by a design rationale with a suggestion for
  tool syntax.

Commmit will be squashed prior to commit.

- Fixed references to GNU properties which have moved to sysvabi64.rst
- Mention in buildattr64.rst that SysVr4 platforms are recommended to
  use GNU properties in loadable units.
- Change lattice to partial order.
- Be more consistent with relocatable object file, rather than
  relocatable file, relocatable object or relocatable object file.
- Rewrite example of library selection to be clearer.

This was unfortunately missed out in the initial commit.

Make further corrections, these were all non-functional in nature.
This commit will be squashed prior to merging.

* In assembly syntax use a comma in [,optional].
* Change document status to Beta quality.
* Remove superfluous Encoding from start of sentence.
* Clarification of "optional" in public subsection header.
* Clarification that public subsections define their own attributes.
* Clarify static linker responsibilities with mixed build attributes.
  and .note.gnu.property sections.
* Reformat the aeabi-pauthabi section to match aeabi-feature-and-bits.
* Move and expand the tool interface section from the rationale to
  the specification.

* Changed assembler syntax to make parameters mandatory and to define
human readable names.
* Generalise mapping of aeabi-feature-and-bits to .note.gnu.property
* Renumber aeabi-feature-and-bits tags to start from 0.