init: tweak Zephyr .init* section name conflicting with toolchain

In case of ARC MWDT toolchain and C++ support enabled the Zephyr .init*
section conflicts with .init* sections derived from toolchain libs.

Let's add 'z_' prefix to Zephyr .init* section (and therefore .device
section as they share the same macros) to make Zephyr section name
unique.

Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>