manifest: track 4.8 toolchains from AOSP

 These were added only a few weeks ago to AOSP. Only ref master exists for them, not tagged yet.
 I am guessing they are just experimenting with these, and contemplating moving to 4.8 as default
 by the next major Android verison release.
 We can track them, just in case someone wants to build kernel using the latest gcc.

 I tested one of my kernel trees to build with this, so having this tracked will help foster early
 adoption to them.

Signed-off-by: Arnav Gupta <championswimmer@aokp.co>

Conflicts:
	default.xml

Change-Id: Ia65b2e183ae0881e52457c94b5083f167d8a95ed