Please bring back the compiler-rt subproject tarball

compiler-rt lets me build compiler builtin functions by just pointing my C compiler at the builtin's source code, this has been EXTREMELY useful to me when building minimal toolchains, especially for iOS toolchains since the iOS compiler-rt build seems to require a Mac, and for custom libcs. The full llvm-project tarball is over 60 times the size of just the compiler-rt tarball. If the compiler-rt subproject tarball isn't brought back I'll probably setup some repo that generates them myself.

Start by locating the historical compiler-rt subproject tarball generation and release entry points, then compare them with the current full llvm-project tarball process. Done means a separately downloadable compiler-rt tarball is produced again and supports the minimal-toolchain and iOS use cases described in the issue.

Written by the indexing model from the issue text.