implement ctfe yl2x and yl2xp1

This PR intriduces builtin compile-time yl2x and yl2xp1 for some platforms.
Supported platforms: DMC, VS (x86), MinGW (x86 and x86_64), GCC (x86, x86_64).
VS (x86_64) doesn't support inline assembler, so I can't implement yl2x for it.
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.