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.
Actually, rndtol may be CTFE-able in GDC - I'll have to check this in detail.