Add mutation test simulation to gccn't

[MattWindsor91]

gccn't is c4t's compiler failure simulator; as such it is arguably a better testbed for new features involving compilation failure than llvm.

As such, this issue tracks adding a mode to gccn't such that:

• whenever C4_MUTATION is greater than 0, it prints 'MUTATION SELECTED: x' where 'x' is the mutation number;
• whenever C4_MUTATION is a multiple of x (where x is user defined), gccn't prints 'MUTATION HIT: 'x';
• whenever C4_MUTATION is a multiple of y (where y is user defined), gccn't crashes.

This should exercise observing both kills and hits.