Test building the toolchain on all supported platforms for building
it (linux, macOS, msys2). Produce cross-built standalone toolchains
that run on Windows for all 4 supported architectures.
Run scheduled builds (daily), built with the latest git versions of
most components. For scheduled builds, the linux toolchain is built
with assertions enabled, as latest git version of llvm occasionally
can have regressions, and many such are caught by having assertions
enabled and building enough code that exercise many code generation
cases.