Panic on stack protector crash instead of exit (#2955)

Newlib has a stack protector fail handler which write(2)s a
message to STDERR which doesn't work here.  Override with a
call to panic().

Tell GCC to protect all functions for stack protection instead of
ones that it heuristically decides need protection.  Slower but
safer, and only when stack protection is enabled.

Author: earlephilhower