Optional stack smash protection -fstack-protector

Add a menu to enable GCC's built-in stack smash protection.  When a
subroutine goes past its end of stack, generate a crashdump on function
exit like:

````
GCC detected stack overrun
Stack corrupted, stack smash detected.

>>>stack>>>

ctx: cont
sp: 3fffff20 end: 3fffffc0 offset: 0000
3fffff20:  40202955 00000001 0000001c 4020287e
3fffff30:  feefeffe 000000fd 00000000 00000000
...
<<<stack<<<
````

Disabled by default because there is a small per-function code overhead
(and CPU time if the function is called very frequently and is very
small).

BearSSL and LWIP are not built using stack smash detection, yet.

Author: earlephilhower