arm: tests: kernel: fix bug in fatal_exception test

In ARM architectures the entry_cpu_exception_extend calls
svc #0 when trying to generate a `K_ERR_CPU_EXCEPTION`, however
z_arm_svc calls z_do_oops with a stack frame only, and gets the
reason from `r0`. This means that the test working was just lucky
and running it with another compiler (or setting the value of r0
before the svc #0 call, made the test fail).

Cortex-A/R 32-bit architectures was doing a BKPT, this works better
but will not be a hard exception when debugger is attached.

I switched all the Cortex 32-bits to the ARM specified undefined
instruction.

Also RISC-V has a designated unimp instruction that should be used to
guarantee trap.

Signed-off-by: Robin Kastberg <robin.kastberg@iar.com>