Sync 4.6 toolchain in prebuilt.

- kernel.mk using 4.6 toolchain from prebuilt folder not prebuilts.
- https://github.com/PAC-man/android_build/blob/pac-4.4/core/tasks/kernel.mk#L151
- fix this error : http://gw.gd/RXQw

Change-Id: I573ba1492d021dd8014aa1eb74996d78da468262