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

(From OE-Core rev: 307902aa05c4537cb34fffe768df1547fb6a12d8)

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

meta/classes-recipe/kernel-arch.bbclass

@@ -81,4 +81,4 @@ KERNEL_AR = "${HOST_PREFIX}ar ${HOST_AR_KERNEL_ARCH}"
 KERNEL_OBJCOPY = "${HOST_PREFIX}objcopy ${HOST_OBJCOPY_KERNEL_ARCH}"
 # Code in package.py can't handle options on KERNEL_STRIP
 KERNEL_STRIP = "${HOST_PREFIX}strip"
-TOOLCHAIN ?= "gcc"
+TOOLCHAIN = "gcc"