stackprotector: ascii armor the stack canary

[riel]

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 https://github.com/thestinger/linux-hardened/