default.xml: comment out all GCC that isn't ARM

This slims our source code and only syncs what GCC toolchains we need. 
 
 - "prebuilts/gcc/linux-x86/arm/arm-eabi-4.X" (kernel)
 - "prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.X" (ROM)

I left the host GCC because I'm not sure if it's needed or not. If it isn't, then please comment it out.