It is already compiled and runs on ARM hardware - phones - and since apps are universal they're inherently capable of adjusting to different screen sizes.
Windows is getting compiled several times a day for both ARM and x86...
Besides only a very small fraction of apps are universal. Also universal apps are not as universal as you think. If you compile an "universal app" there are 5 different programs generated:
1) Windows Phone ARM (Phone layout) -> for ARM Phones
2) Windows Phone x86 (Phone layout) -> for x86 phones and phone emulator
3) Windows ARMv7 (Tablet layout) -> for Windows RT machines
4) Windows 32 bit (Tablet layout) -> for x86 machines and Windows emulator
5) Windows 64 bit (Tanlet layout) -> for x64 machines
It is just the store, which downloads the correct version for you. The Phone and Windows versions dont use 100% the same code either. All the 3 Windows versions using 100% the same code and XAML.
Also there is currently no compiler for ARMv8 available.
This way MS doesn't have to support one more platform, as it's the same mobile SKU
Windows RT is not "one more platform". It is just Windows. Both, Windows 10 mobile (Windows 10 Phone) and full Windows 10 can be compiled for both ARM and x86. It's not that one SKU can only be compiled for one architecture.