llvm: build pure llvm toolchain + zig: enable mingw64 & ucrt64

[raedrizqie]

• builds a pure llvm toolchain on top of MinGW
• builds libc++ and libunwind with builtin compiler-rt on all systems
• emulated TLS is disabled when building with Clang
• remove dependency of libgcc_s and libpthread on non Clang systems
• fix building of libc++abi shared library

Notes: We previously build with -DLIBCXXABI_ENABLE_STATIC_UNWINDER=ON which actually leads to libc++ not depends on libunwind, with both libc++ and libunwind exporting the same unwind functions.

If we set -DLIBCXXABI_ENABLE_STATIC_UNWINDER=OFF, then libc++ will correctly depends on libunwind.