Require the STACK option for code that captures stacks of running threads.

stack_machdep.c is compiled if either of the DDB or STACK options is
specified, but stack_save_td_running() isn't useable from DDB. Moreover,
stack_save_td_running() works by raising an NMI on the CPU running the
target thread, and the corresponding handler is compiled only if STACK is
configured.

Reported by: kib
MFC after: 1 week