kernel-arch.bbclass: Do not use weak assignment for TOOLCHAIN

kernel-arch.bbclass: Do not use weak assignment for TOOLCHAIN

If a distro sets TOOLCHAIN with ?= or = then it overrides this
too, however, we demand GCC to be in there due to KERNEL_CC etal
Make it hard assignment, it can still be overturned via a bbappend
in that case it is intentional to use non-gcc compiler for kernel

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>