added NORETURNs to functions that should not return
and added `UNREACHABLE();` to hint the compiler unreachable lines

added right signature for first parameter of `thread_stack_init()`
added `UNREACHABLE();` macro to `cpu/lpc1768/atom.c` and `cpu/msp430-common/cpu.c`
Merge pull request #1079 from BytesGalore/add_NORETURNs

core: and cpu: Added NORETURN to functions that should not return