Zero out the first byte of the stack canary value on 64 bit systems,
in order to mitigate unterminated C string overflows.
The null byte both prevents C string functions from reading the
canary, and from writing it if the canary value were guessed or
obtained through some other means.
Reducing the entropy by 8 bits is acceptable on 64-bit systems,
which will still have 56 bits of entropy left, but not on 32
bit systems, so the "ascii armor" canary is only implemented on
64-bit systems.
Inspired by the "ascii armor" code in execshield and Daniel Micay's
linux-hardened tree.
Also see https://github.com/thestinger/linux-hardened/
v2:
- improve changelogs
- address Ingo's coding style comments
