tests: error_hook: Workaround for RX architecture

GCC for RX would crash with an internal compiler error during
DWARF frame generation in `dwarf2out_frame_debug_adjust_cfa()`
when compiling the `trigger_fault_illegal_instruction()`
test function.

This patch adds `__builtin_unreachable()` to help the compiler
reason about control flow, preventing incorrect DWARF CFA
generation.

Signed-off-by: Duy Nguyen <duy.nguyen.xa@renesas.com>