Cosmopolitan libc platform support

At https://justine.lol/cosmo3/ we can learn about the progress of the cosmopolitan libc, which is a:

Cosmopolitan Libc makes C a build-anywhere run-anywhere language, like Java, except it doesn't need an interpreter or virtual machine. Instead, it reconfigures stock GCC and Clang to output a POSIX-approved polyglot format that runs natively on Linux + Mac + Windows + FreeBSD + OpenBSD + NetBSD + BIOS on AMD64 and ARM64 with the best possible performance.

A toolchain can be downloaded from Index of /pub/cosmocc/.

CMake doesn't support Cosmopolitan as a platform, see Support for "cosmopolitan libc" (#25578) · Issues · CMake / CMake · GitLab (kitware.com) but one could have a toolchain that can compile Qt base attached to this ticket.

cosmo.cmake