AArch64 build attributes specification

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.

buildattr64/CONTRIBUTIONS

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+Contributions to this project are licensed under an inbound=outbound
+model such that any such contributions are licensed by the contributor
+under the same terms as those in the LICENSE file.

buildattr64/LICENSE

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+This work is licensed under the Creative Commons
+Attribution-ShareAlike 4.0 International License. To view a copy of
+this license, visit http://creativecommons.org/licenses/by-sa/4.0/ or
+send a letter to Creative Commons, PO Box 1866, Mountain View, CA
+94042, USA.
+
+Grant of Patent License. Subject to the terms and conditions of this
+license (both the Public License and this Patent License), each
+Licensor hereby grants to You a perpetual, worldwide, non-exclusive,
+no-charge, royalty-free, irrevocable (except as stated in this
+section) patent license to make, have made, use, offer to sell, sell,
+import, and otherwise transfer the Licensed Material, where such
+license applies only to those patent claims licensable by such
+Licensor that are necessarily infringed by their contribution(s) alone
+or by combination of their contribution(s) with the Licensed Material
+to which such contribution(s) was submitted. If You institute patent
+litigation against any entity (including a cross-claim or counterclaim
+in a lawsuit) alleging that the Licensed Material or a contribution
+incorporated within the Licensed Material constitutes direct or
+contributory patent infringement, then any licenses granted to You
+under this license for that Licensed Material shall terminate as of
+the date such litigation is filed.

buildattr64/README.md

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+<div align="center">
+   <img src="Arm_logo_blue_RGB.svg" />
+</div>
+
+# Build Attributes for the Arm® 64-bit Architecture (AArch64)
+
+## About this document
+
+The [Build Attributes for the Arm 64-bit Architecture](buildattr64.rst) describes the
+
+## About the license
+
+As identified more fully in the [LICENSE](LICENSE) file, this project
+is licensed under CC-BY-SA-4.0 along with an additional patent
+license.  The language in the additional patent license is largely
+identical to that in Apache-2.0 (specifically, Section 3 of Apache-2.0
+as reflected at https://www.apache.org/licenses/LICENSE-2.0) with two
+exceptions.
+
+First, several changes were made related to the defined terms so as to
+reflect the fact that such defined terms need to align with the
+terminology in CC-BY-SA-4.0 rather than Apache-2.0 (e.g., changing
+“Work” to “Licensed Material”).
+
+Second, the defensive termination clause was changed such that the
+scope of defensive termination applies to “any licenses granted to
+You” (rather than “any patent licenses granted to You”).  This change
+is intended to help maintain a healthy ecosystem by providing
+additional protection to the community against patent litigation
+claims.
+
+## Defects report
+
+Please report defects in the [Build Attributes for the Arm 64-bit Architecture](buildattr64.rst) to the [issue
+tracker page on
+GitHub](https://github.com/ARM-software/abi-aa/issues).

buildattr64/TRADEMARK_NOTICE

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+The text of and illustrations in this document are licensed by Arm
+under a Creative Commons Attribution–Share Alike 4.0 International
+license ("CC-BY-SA-4.0”), with an additional clause on patents.
+The Arm trademarks featured here are registered trademarks or
+trademarks of Arm Limited (or its subsidiaries) in the US and/or
+elsewhere. All rights reserved. Please visit
+https://www.arm.com/company/policies/trademarks for more information
+about Arm’s trademarks.