kernel: fix KERNEL_TOOLCHAIN_PATH_gcc and usage

Previously, this would be a full compile prefix
(e.g. prebuilts/gcc/.../bin/aarch64-linux-gnu-). Since we're adding
this to PATH, it only needs to be prebuilts/gcc/.../bin.

Also remove the extra /bin/ added by kernel.mk.

Change-Id: If2490729128c0243c61de646f6c8e19a15c8dc1c
2 files changed