Build Tor and Tor Browser with -mmitigate-rop

GCC 6 has a new option, -mmitigate-rop, which modifies the generated code to make finding ROP gadgets a bit harder. This is not CFI and does not provide strong protections, but it's better than nothing and is easier to use than alternatives, given that it doesn't require modifying source code for compatibility or loading a new runtime.

-mmitigate-rop Try to avoid generating code sequences that contain unintended return opcodes, to mitigate against certain forms of attack. At the moment, this option is limited in what it can do and should not be relied on to provide serious protection.

I suppose someone should try compiling Tor with this and scan for ROP gadgets using popular ROP compilers on it.