classes: make TOOLCHAIN more permissive for kernel

Currently TOOLCHAIN is strictly set to gcc in kernel-arch.bbclass.
And this prevents any TOOLCHAIN changes for any kernel recipe.
This change makes TOOLCHAIN configurable as usual.

Signed-off-by: Alexey Smirnov <pyih.soft@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>