I combined 2 open source projects. A javascript x86 emulator and a GBA emulator. The X86 VM runs tinycore linux with the devkitARM toolchain and a script transfers the compiled .gba file to the emulator using an emulated serial port.
I did this because i wanted to see how slow would it really be. (it takes a minute to compile a simple example on my i5 t420)
https://hackaday.com/2017/06/20/writing-a-new-game-for-the-game-boy-color/