kernel/linux: fix fall-out due to arm64 special case

Cset 3b61be3d7aa6 (prepare for arch whose kenel name is not the standard name)
failed to name a variable consistently, so all archs but arm64 were broken.

Fix that by renaming the variable in a consistent way.

Reported-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

# windows config options

## depends on ARCH_x86

##

## select WINDOWS

##

## help Build a toolchain targeting systems running Windows as host