kernel: Use linux-android- toolchains to build kernel on aarch64 and x86

 * -androidkernel ones became symlinks to -android ones long time ago, use
   -android executables directly to simplify the logic on these platforms.
 * Keep arm on -androidkernel because it doesn't boot for some reason.

Change-Id: I97dd93121fc0e805a965e3f8b84466dd7bd686c0