Add __longjmp_chk()

When compiled with -D_FORTIFY_SOURCE=1 (this is the default in Ubuntu),
a program that uses longjmp() can end up calling __longjmp_chk() instead.
This patch adds a trivial implementation of this missing function.

Ideally, __longjmp_chk() should do some sanity checks on the "env" we're
jumping to, namely that it actually points to a stack frame earlier on
our stack. This patch is functionally-correct, but includes no such checks.
Glibc does some checks, but they are also far from being bulletproof (they
do little more than checking the env address is higher than the current
stack pointer).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>