arch: riscv: stacktrace: implement `arch_stack_walk()`

Created the `arch_stack_walk()` function out from the original
`z_riscv_unwind_stack()`, it's been updated to support
unwinding any thread.

Updated the stack_unwind test case accordingly.

Increased the delay in `test_fatal_on_smp`, to wait
for the the fatal thread to be terminated, as stacktrace can
take a bit more time.

Doubled the kernel/smp testcase timeout from 60 (default) to
120s, as some of the tests can take a little bit more than 60s
to finish.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>