manifest: track gcc 4.8 prebuilts from AOSP

while there do not exist any aosp kitkat branch or 4.4 tags for
  these repos yet, but tracking them would give developers the
  option to build with 4.8 instead of current 4.7
  in the process bugs in the trees that conflict with gcc 4.8 will
  get ironed out, and it will foster early adoption of a new
  toolchain