Add AROS aarch64 build target

[midwan]

Summary

DOpus5 can now be cross-compiled and packaged for AROS aarch64 (ABIv11, Raspberry Pi 3/4/5 64-bit) through the same make and release workflow as the existing AROS targets. The resulting program, library, modules, commands, and .lha archive are native AROS AArch64 ELF artifacts.

This consumes the toolchain published by BlitterStudio/aros-compiler-docker#9.

Related: BlitterStudio/aros-compiler-docker#9

Changes

• Add make aarch64-aros to the root, Modules, and Misc/C dispatchers.
• Use DWARF debug information for aarch64; its GCC backend does not support STABS. The override also works for direct component builds.
• Make DEBUG=0 consistently mean a release build for logging and platform identity, producing [AROS-aarch64] / [AROSdev-aarch64] version strings.
• Mark PROGRAM_ENTRIES as intentionally handled by custom -nostartfiles library/module startup, matching AROS's shared-library contract.
• Report ARM64 and FPU instead of falling through to m68k CPU/FPU detection.
• Add the published midwan/aros-compiler:aarch64-aros image to the debug/release matrix and release artifact set.
• Document the target and current limitation: sftp=yes is unavailable because the aarch64 SDK does not yet ship libssh2.

Affected platforms

• AmigaOS 3 (os3)
• AmigaOS 3 / 68000 (os3-68000)
• AmigaOS 4 (os4)
• MorphOS (mos)
• AROS i386 (i386-aros)
• AROS x86_64 (x86_64-aros)
• AROS aarch64 (aarch64-aros)

Verification

• Published midwan/aros-compiler:aarch64-aros image:
  • Full debug build: passed; DirectoryOpus reports [AROSdev-aarch64].
  • Full optimized build and release packaging: passed; produced DOpus5_102_aarch64-aros.lha and [AROS-aarch64] binaries.
  • DirectoryOpus, dopus5.library, and xadopus.module: ELF 64-bit LSB, ARM aarch64, AROS OS ABI.
  • Root and direct Modules/Misc entrypoints propagate -g; no unsupported-STABS or unknown-architecture warning.
• x86_64 regression build: passed with its existing -gstabs behavior.
• x86_64 release/debug platform-token preprocessing: [AROS-x86_64] / [AROSdev-x86_64].
• Workflow YAML parse and git diff --check: passed.
• Structured code review (DopusCodeReview): complete, no residual findings.

Post-Deploy Monitoring & Validation

• Window / owner: repository maintainer watches this PR's matrix and the first master/tag build after merge.
• Healthy signals: build-aarch64-aros-debug and build-aarch64-aros-release succeed; upload contains DOpus5_*_aarch64-aros.lha; no stabs or PROGRAM_ENTRIES link error appears.
• Failure signals: aarch64 compile/link failure, missing release artifact, or Docker image pull failure.
• Mitigation: revert the aarch64 matrix/target commit while keeping the published compiler image available for diagnosis; no runtime data migration is involved.

Checklist

• CI is green (or expected failures are explained).
• The relevant platform build(s) complete without new warnings or errors.
• New files carry the correct project licence header.
• Compiler warnings introduced by the change have been addressed.
• User-visible documentation is updated where needed.
• ChangeLog has a top-of-file entry describing the change.
• Relevant version/revision constant(s) are bumped, or the PR explains why no bump is required (new target at existing revisions; existing target versions unchanged).
• No drive-by reformatting of unrelated code.

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