Fix toolchain Docker build: pre-fetch Nanvix OS artifacts

[ppenna]

Problem

The toolchain image build fails at the GCC stage 0 configure step:

ERROR: Failed to download Nanvix release. Ensure 'gh' is installed and authenticated.
[ERROR] Failed to download dependencies
[ERROR] Configuration failed

The GCC z script on the v13.3.0 branch added a step in download_steps() (run during ./z configure) that downloads the public nanvix/nanvix OS release (v0.13.17) via gh release download to stage libposix.a, user.ld, and the nanvixd binaries into the install prefix. gh is not installed or authenticated inside this Docker builder, so the build fails. (The nanvix/gcc and nanvix/newlib CI builds succeed because GitHub-hosted runners ship gh with a token.)

Fix

Stage the same Nanvix OS artifacts into ${PREFIX} with curl before building GCC. The z script then detects them as already present and skips the gh download:

Nanvix OS libraries already present in '/opt/nanvix/lib/'.

The release tag, asset name and SHA-256 mirror the constants in nanvix/gcc's z script, and are exposed as ARGs for easy maintenance. No build secrets or auth required (the asset is public).

Validation

Built the image locally with the latest published component tags:

• BINUTILS_COMMIT=nanvix-v2.42-686c3b0673c
• GCC_COMMIT=nanvix-v13.3.0-e795b98ca52-stage1
• NEWLIB_COMMIT=nanvix-v4.4.0-c3cdbe37a-gcc

Results:

• Full build succeeds (binutils -> GCC stage 0 -> newlib -> GCC stage 1).
• Runtime smoke test passes: i686-nanvix-gcc (GCC) 13.3.0, i686-nanvix-g++ (GCC) 13.3.0, GNU assembler/ld (GNU Binutils) 2.42.
• CI-equivalent cross-compile test: i686-nanvix-gcc compiles tests/hello.c to a valid i686 ELF32 EXEC executable.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[ppenna]

Superseded by nanvix/gcc#58, which decouples the Nanvix OS artifact download from z configure (the proper upstream fix). The toolchain Dockerfile no longer needs the curl pre-fetch hack.