xtensa: context returns to thread after kernel oops

[andrewboie]

Describe the bug
See CI failures for #15032

What is happening is that a thread is blowing its stack which generates a call to z_except_reason(). k_thread_abort() gets called and the thread should never return to its faulting context.

Instead, the thread returns to where it faulted! After the "Caught system error" observe that the thread continues to run, failing the test case.

test stack sentinel overflow - timer irq
posting 1024 bytes of junk to stack...
waiting for tick advance...
@ /home/apboie/projects/zephyr3/zephyr/kernel/thread.c:259:
***** Stack Check Fail! *****
Current thread ID = 0x60005b78
Faulting instruction address = 0xdeaddead
Thread 0x60005b78 Caught system error -- reason 2
FAIL - stack_sentinel_timer@160. should never see this

    Assertion failed at /home/apboie/projects/zephyr3/zephyr/tests/kernel/fatal/src/main.c:207: check_stack_overflow: (rv equal to TC_FAIL)
thread 0x60005b78 was not aborted

[andrewboie]

Some more details:

1. The timer interrupt fires. On the way out of the timer interrupt, xtensa_excint1_c() in the xtensa arch code makes a call to z_get_next_switch_handle().
2. z_get_next_switch_handle() in kernel/sched.c calls z_check_stack_sentinel()
3. Since we blew the stack, z_check_stack_sentinel() calls z_except_reason(_NANO_ERR_STACK_CHECK_FAIL)
4. xtensa has no arch-specific Z_ARCH_EXCEPT() implemented, so a real exception is not generated. Instead z_except_reason() is the default implementation in include/kernel.h which just calls z_NanoFatalErrorHandler() directly
5. z_NanoFatalErrorHandler() runs, calls z_SysFatalErrorHandler() which is a custom implementation in tests/kernel/fatal/src/main.c
6. z_SysFatalErrorHandler() calls k_thread_abort()
7. It appears the z_reschedule() call in z_impl_k_thread_abort() returns to the caller on Xtensa. The rest of the interrupt unwinds and then the faulting thread is run again even though it shouldn't be.

My guess is that due to some implementation detail on xtensa, trying to call z_except_reason() in that particular context doesn't have the desired effect on this arch.