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.
```
> cat test.d 
import std.math : rndtol;
static x = rndtol(1.2);
pragma(msg, rndtol(1.2));

> gdc test.d -S
1L
```