stackprotector: ascii armor the stack canary

From: riel
Date: Fri May 19 2017 - 17:37:11 EST

Zero out the first byte of the stack canary value on 64 bit systems,
in order to prevent unterminated C string overflows from being able
to successfully overwrite the canary, even if an attacker somehow
guessed or obtained the canary value.

Inspired by execshield ascii-armor and PaX/grsecurity.

Thanks to Daniel Micay for extracting code of similar functionality
from PaX/grsecurity and making it easy to find in his linux-hardened
git tree on