toolchain: add toolchain for AArch64

[jmaksymowicz]

Description

Added toolchain for compiling code to AArch64 (Arm in 64-bit mode) and scripts for building Phoenix RTOS for ZynqMP platform.

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

7 958 tests  ±0   7 416 ✅ ±0   40m 13s ⏱️ -59s
  470 suites ±0     542 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit a3b9107. ± Comparison against base commit 4948ac9.

♻️ This comment has been updated with latest results.

[badochov]

This entry as well as one in ld/Makefile.in are redundant.

THey are only needed if we provide a new linkers configuration (.sh files, eg ld/emulparams/elf32_sparc_phoenix.sh).

On other platforms we provide those configs even though often times they are redundant. IMHO using the standard config is good enough for now.

[badochov]

What would you say about adding phoenix target to the targets below (aarch64*-*-elf | aarch64*-*-fuchsia* | aarch64*-*-rtems*) )?