cmake: initialize ARCH for native builds
cmake: initialize ARCH for native builds

We need ARCH, because it needs to be set for ARM7, ARM6 to be
initialized.

Strangely, on different machines (both ARMv7, Arch), ${ARCH}
var is either empty or 'native'. Handle both cases.

eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero