ISPC build dependencies

This repo is hosting ISPC build dependencies to enable fast and reliable downloads in Github Actions CI. The binaries are hosted in this project's Releases.

Specifically, the repo contains:

1. LLVM builds, which are built by alloy.py script form ISPC repo. The builds are produced by rebuild-llvm*.yml pipelines, which can be found here. These are vanilla LLVM builds configured to support x86, ARM and WASM targets. For some versions patches backported from LLVM trunk are applied, when there are known bugs affecting ISPC, all these patches are applier by alloy.py automatically and all these patches are located in ISPC repo llvm-patches folder. The license for LLVM project can be found here. The source code for LLVM project used for the builds is located here.
2. GnuWin32 tools needed for cross platform build on Windows. This is a mirror of SourceForce location and contains the binary and source code. The license for the code can be found in the source code package.