Wire everything that relies on stack smashing detection to call
`__stack_chk_fail()` (aka what libssp / ssp / stack-protector uses)
Expose it in our debugging header

Rename overflow -> smashing, as these are different things we are trying
to detect (meaning, that we check for things writing there, not some
kind of `alloca` issue or the way `-fstack-check` would have worked)
ref. esp8266#8666

`-fstack-protector` continues to work as it always did
CONT replaces `abort()`, also moves its check to the loop wrapper to
avoid dumping otherwise useless SYS context memory
StackThunk replaces a similar `abort()` call