arch: riscv: stacktrace: conditionally check stack_info

Check if an address is in the thread stack only when
`CONFIG_THREAD_STACK_INFO` is enabled, since otherwise the
`stack_info` will not be available.

This fixes compilation error when `CONFIG_THREAD_STACK_INFO`
is explicitly disabled.

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