Add source-built legacy ARMv6 big-endian CI build

[demoray]

Summary

Adds an ARMv6 big-endian build path for legacy BE32/EABI4 Linux devices.

The build now uses a repo script instead of embedding the logic in GitHub Actions. The script builds an armeb-linux-musleabi toolchain from pinned musl-cross-make sources, builds both full and minimal AVML binaries, patches the ELF EABI version to EABI4, and verifies the resulting artifacts.

Key details

• Builds the cross toolchain from source instead of downloading prebuilt musl.cc artifacts.
• Uses a custom Rust target spec pinned to ARMv5TE codegen to avoid BE8 instruction encoding.
• Forces static non-PIE output for old kernels that cannot exec static PIE / ET_DYN.
• Adds a linker wrapper and musl compatibility shim needed by Rust std and OpenSSL.
• Verifies each artifact is:
  • big-endian
  • ET_EXEC
  • EABI4
  • no BE8 flag
  • no dynamic interpreter
  • Tag_CPU_arch: v5TE
  • no movw / movt instructions

Validation

• Ran eng/build-armv6b.sh with a clean source-built toolchain cache.
• Confirmed both artifacts were produced:
  • target/armeb-unknown-linux-musleabi/release/avml
  • target/armeb-unknown-linux-musleabi/release/avml-minimal