arch: riscv: use symtab to print function name in stack trace

Selecting `CONFIG_EXCEPTION_STACK_TRACE_SYMTAB` will
enable the symtab generation which will be used in the
stack trace to print the function name of the return
address.

Updated the `stack_unwind` test to test the symbols in a
stack trace.

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