Fix undefined behaviour in vilogbk_vi_vd (#355)

C standard do not define have left shift should affect the sign bit, which results in the following runtime error if `vilogbk_vi_vd` is compiled by clang with sanitizer checks enabled:
```
sleef/src/libm/sleefsimddp.c:329:49: runtime error: left shift of 4095 by 20 places cannot be represented in type 'int'
```
Can be fixed by explicitly specifying type of shift 1st operand as unsigned: i.e. replacing `1` with `1U`