Rework toolchain building flow

[jwnrt]

• Removes 32-bit host configurations
• Removes GCC from toolchain
• Builds Binutils directly without crosstool-ng
• Removes meson support configurations
• Adds option for changing the build type for Clang (e.g. Debug or Release)

[luismarques]

The build-clang-with-args.sh usage string is missing the corresponding update?

[jwnrt]

I did a release run on my fork here: https://github.com/jwnrt/lowrisc-toolchains/actions/runs/16989098118

Which created this release: https://github.com/jwnrt/lowrisc-toolchains/releases/tag/test-2025-08-15-v2

And I've done an OpenTitan CI run with this release here: https://github.com/lowRISC/opentitan/actions/runs/16990654911/job/48172327473

[luismarques]

./contrib/build-opentitan-dist.sh is useful but it's not ideal for iterative toolchain development/testing. In general, we want something where we can rebuild the toolchain with small changes (e.g. add a printf) and immediately try it in OpenTitan (with the Bazel override). That script provides a good starting point for non-CI builds but it rebuilds more than necessary. It also takes a while to build debug builds because it doesn't use the host LLD to link LLVM (-DLLVM_ENABLE_LLD=True) nor allows enabling other options for iterative development (-DBUILD_SHARED_LIBS=True -DLLVM_USE_SPLIT_DWARF=True). I'd like to see options/alternatives for debug builds (./build-clang-with-args.sh --debug), using Ninja, using the host LLD, avoiding rebuilding binutils, etc. It probably also doesn't need to be inside contrib.