In addition to the Bionic short-comings already mentioned here, there appears to be other work to deal with the fact that things must be cross-compiled in a way that you can build a compiler for your host machine, but generate Android binaries (and for each architecture).
Also, there are also a bunch of support things for libICU which is not an official component on Android we are allowed.
Android uses Bionic libc which has its quirks. The Android kernel also has several patches that were at one time quite controversial, though many have moved upstream to the mainline.
On the other hand the Frameworks are quite expressive for applications, but one needs to use Java 6.5 or resort to third party solutions like retrolambda and Kotlin.
* CMake files are expanded to handle an Android target.
* Code that checks for specific defines or values like __FREEBSD__ || __LINUX__ now have an added condition to also check for __ANDROID__.
* Some tooling scripts to integrate adb into the Swift SDK.
* Some minor fixes elsewhere.
So, special support is a really minimal set of changes that any platform would need.