Windows/Clang: Build system adaptation

* Bump win32-tarballs to 0.7
* Move Windows toolchain autoconf logic into separate file
* Use clang and LLVM utilities as described in #21019
* Disable object merging as lld doesn't support -r
* Drop --oformat=pe-bigobj-x86-64 arguments from ld flags as LLD detects
  that the output is large on its own.
* Drop gcc wrapper since Clang finds its root fine on its own.