Build GCC on CI
Previously, we have downloaded a specific commit of GCC and prebuilt it inside Docker using the `build-gccjit.sh` script. This PR removes that scripts and uses the bootstrap GCC step. This allows us to use the `src/gcc` submodule for determining which GCC should be built, and it also moves the logic closer to LLVM, which is also built by bootstrap.
A few things to note:
- The `sccache` option is currently in the `llvm` block, so the GCC build uses `llvm.ccache`, which is a bit weird :) We could either add `gcc.ccache`, or (what I think would be better) to just move `ccache` to the `build` section, as I don't think that it will be necessary to use ccache for LLVM, but not for GCC.
- When the GCC codegen backend is built, it needs to depend on a step that first builds GCC. This is currently done in a hacky way. The proper solution is to create a separate step for the GCC codegen backend, but that is a larger change. Let me know what you think.
r? `@onur-ozkan`
try-job: i686-msvc-1
try-job: x86_64-mingw-1
a46c755c6a