implement ctfe yl2x and yl2xp1

This PR intriduces builtin compile-time yl2x and yl2xp1 for some platforms.
Supported platforms: DMC, VS (x86, x86_64), MinGW (x86 and x86_64), GCC (x86, x86_64).
Solaris (_sun) may use another compiler (I don't know it).
If platform doesn't support this builtin, "cannot evaluate unimplemented builtin..." error will be raised as it was early.
I don't know. I did is similar examples like `eval_sqrt`
```
Expression *eval_sqrt(Loc loc, FuncDeclaration *fd, Expressions *arguments)
{
    Expression *arg0 = (*arguments)[0];
    assert(arg0->op == TOKfloat64);
    return new RealExp(loc, Port::sqrt(arg0->toReal()), arg0->type);
}
```
Maybe CTFE stores all floating numbers with `op == TOKfloat64`