[CTFE] Implement yl2x and yl2xp1 builtin functions

Need for computing exponent of float number at compile time. This pull allow to implement to real->ubyte[] conversion in CTFE, formatting float number in CTFE et c.
1. The reason for the existence of `yl2xp1` is because of underflow precision problems - but this implementation seems to not be aware of that.
2. Use inline assembler to use actual yl2x and yl2xp1 instructions where possible.
3. The repeated code should be refactored.