Add build configuration for Cosmopolitan Libc, enabling mruby to be
compiled as an "Actually Portable Executable" (APE) that runs natively
on multiple platforms from a single binary.
Supported platforms:
- Linux (x86_64, ARM64)
- macOS (x86_64, ARM64)
- Windows (x86_64)
- FreeBSD (x86_64)
- OpenBSD (x86_64)
- NetBSD (x86_64)
Included binaries:
- mruby.com   - mruby interpreter
- mrbc.com    - bytecode compiler
- mirb.com    - interactive Ruby shell
- mrdb.com    - debugger
- mruby-strip.com - debug info stripper
Usage:
  COSMO_ROOT=~/cosmo rake MRUBY_CONFIG=cosmopolitan
The cosmocc toolchain can be downloaded from https://cosmo.zip/pub/cosmocc/