Discourage use of `-mmitigate-rop` in C and C++ Compiler Hardening Guide

[thomasnyman]

Splitting this off from Dominik Czarnota's extensive feedback in #330.

The -mmitigate-rop option was introduced around GCC 6 and later deprecated in 2018 in favor of control-flow protection.

The idea behind -mmitigate-rop was to avoiding certain opcodes that may inadvertently encode a return instruction in the instruction stream. It did it using two strategies: renaming entire chains of registers, or inserting extra register to register copies if there a free scratch register is available.

Overall benefit of -mmitigare-rop was weak and control-flow protection in modern compilers and hardware offer better (but not complete) protection.

Resources:

• https://gcc.gnu.org/legacy-ml/gcc-patches/2015-11/msg01773.html
• https://security.stackexchange.com/questions/181255/how-does-gccs-mmitigate-rop-work
• https://gcc.gnu.org/pipermail/gcc-patches/2018-August/504637.html

[thomasnyman]

Addressed by #608