fix: resolve size_t ABI mismatch between clang and GCC toolchain

[ppenna]

Summary

Fixes the size_t ABI mismatch where clang mangles size_t as unsigned int (j) but the GCC toolchain's libstdc++/libsupc++ uses long unsigned int (m), causing linker failures for any C++ code targeting i686-nanvix.

Approach

Updates LLVM_COMMIT to pick up the upstream fix from nanvix/llvm-project#17 which ships libc++, libc++abi, and libunwind built with clang as stage 1 runtimes. Since these libraries are compiled with clang, their ABI matches clang's size_t definition.

Changes

• Dockerfile: Bump LLVM_COMMIT to 405181e (includes stage 1 libc++ runtimes); update stage 1 comment; add smoke test verifying libc++.a, libc++abi.a, and libunwind.a are present
• tests/test_size_t_abi.cpp: New C++ regression test that exercises ::operator new(size_t) — the exact pattern that was broken
• build.yml: Add CI step to compile and link the C++ test with -stdlib=libc++, then verify ELF output (added to both build and registry smoke test jobs)
• README.md: Update pinned LLVM commit

Validation

• LLVM commit 405181e verified to exist and includes the libc++ runtimes fix
• Dockerfile passes docker build --check (syntax validation)
• Workflow YAML passes schema validation
• C++ test specifically targets the operator new(size_t) ABI mismatch from the issue reproduction

Closes #9