kernel: Set kernel toolchain roots for aarch64, arm, and x86

The GCC toolchains are no longer added to the path by default, so
we need to hardcode the full paths for these. Also set variables
for every arch, and then set the toolchain to the arch specific var.
This is so we can set the ARM32 cross compile flag for vDSO32
enabled kernels.

Change-Id: I7e099a26a17f4a3cc9be6232a26c671e8fcfbdef