arch: arm: do not add CODE_UNREACHABLE in Z_ARCH_EXCEPT

For ARM, Z_ARCH_EXCEPT triggers an SVC to induce a system error.
This code block may be inlined, so, if we want to return from
this error DIRECTLY to thread mode, e.g. if the system error
occurred in ISR context and we are not aborting the current
thread, we must instruct the compiler that the execution
may continue after the inlined SVC. Therefore, we must remove
the CODE_UNREACHABLE statements.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>