gcc 4.8 added to the repo

Hi, I've added gcc 4.8 to the prebuilts repo -- if you want to start using it, simply replace 4.7 in TARGET_TOOLS_PREFIX with 4.8.

I've updated the Galaxy Nexus build to use 4.8 for a start - so far, it's running nicely, except the software keyboard doesn't work. (Workaround: in development builds, the alternative Hackers' keyboard is included, that one works fine. Interestingly it's based on the same code originally.)

ttyl bero