arch/Config.in: introduce BR2_KERNEL_64_USERLAND_32 symbol

On some architectures one can be running a 64-bit kernel with a 32-bit
userland. Such is the case for sparc64 (unsupported) for example and
mips64 with n32 ABI.

Some tools that interface directly with the kernel need to be built
specially for this, so introduce this symbol to tweak their build in
one central kludge to be future-proof.

Example: bug #6602.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>