The AT9155800 is an ARM-based microcontroller, and is completely unrelated to the AVR (except obviously that Atmel makes them both). You are looking for a gcc ARM cross compiler. You can either build it yourself (it's standard for gcc), or get hold of pre-built binaries for the OS of your choice. Now that you know what you are looking for, googling for "gcc ARM windows", for example, will get you started.