scripts: subarch.include: fix SUBARCH on macOS hosts

When building the Linux kernel on an aarch64 macOS based host, if we don't
specify a value for ARCH when invoking make, we default to arm and thus
multi_v7_defconfig rather than the expected arm64 and arm64's defconfig.

This is because subarch.include invokes `uname -m` which on MacOS hosts
evaluates to `arm64` but on Linux hosts evaluates to `aarch64`,

This allows us to build ARCH=arm64 natively on macOS (as in ARCH need
not be specified on an aarch64-based system).

Avoid matching arm64 by excluding it from the arm.* sed expression.

Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Suggested-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>