armclang: adding toolchain/armclang.h header

The armclang.h includes the toolchain/llvm.h header.

Also it redifines the __GENERIC_SECTION and Z_GENERIC_SECTION so that
they includes the `used` attribute which is needed by armlink.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>