Android: Add support for natively building on Android
A bit of background first, the Termux app for Android presents a terminal and package manager for Android, which has included a native Android CMake package from Termux's inception more than four years ago. However, CMake checks utsname to determine CMAKE_HOST_SYSTEM_NAME and it would represent an Android host as "Linux", so CMake would treat an Android-AArch64 device as a Linux-AArch64 host for builds. This worked but quickly got annoying, so I finally patched CMake a couple months ago and got it merged in the Termux package repo, which I'm now submitting here, !3917 (merged), so Termux doesn't have to maintain this.