Use mingw-w64/clang toolchain to build Rust

Trac:
Parent Ticket: legacy/trac#29318 (moved)

added component::applications/tor browser in Legacy / Trac gitlab-tb-tor-browser-build in Legacy / Trac owner::tbb-team in Legacy / Trac parent::29318 in Legacy / Trac priority::medium in Legacy / Trac severity::normal in Legacy / Trac status::new in Legacy / Trac tbb-rbm in Legacy / Trac type::defect in Legacy / Trac labels

Or even https://github.com/rust-lang/rust/issues/58713 Or even lld-link ;) with 32-bit Rust's SEH-based unwinding: https://github.com/rust-lang/rust/issues/39915#issuecomment-330058429

Add magic gitlab keyword.

Trac:
Keywords: N/A deleted, gitlab-tb-tor-browser-build added

mentioned in issue legacy/trac#29318 (moved)

moved from legacy/trac#29320 (moved)

added Bug label and removed 1 deleted label

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added Backlog label

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marked this issue as related to #29318 (closed)

We're not yet there, but close.

We arrive to linking, but it fails because it cannot find libgcc and libgcc_s:

error: linking with `x86_64-w64-mingw32-clang` failed: exit status: 1
  |
  = note: "x86_64-w64-mingw32-clang" [very long command snipped]
  = note: lld: error: unable to find library -lgcc_s
          lld: error: unable to find library -lgcc
          clang-14: error: linker command failed with exit code 1 (use -v to see invocation)

However, LLVM 15 will include a new project: llvm-libgcc. But I am not sure they will help, see also this GitHub issue.

I wonder if we can compile only these libraries we need, without compiling the full GCC toolchain.

Another alternative is considering gnullvm targets, but they're tier 3 and 64-bit only.

Another alternative is considering gnullvm targets, but they're tier 3 and 64-bit only.

They've been promoted to Tier 2 without Host Tools and include also i686 (and aarch64) .

128 might be the time we switch to these targets and complete #29318 (closed).

(For the record, I checked because I had reproducibility problems due to Rust, and they might have been caused by mingw-w64, which we don't build in a reproducible way).