Add several functions to knc.h

New functions:
- __scatter64_i64
- __scatter_base_offsets32_double
- __scatter_base_offsets32_i64
- fixed 'cast_trunc', 'cast_zext' and 'cast_sext' implementations (the previous was faulty)
- added '__select (i8/16)', '__equal_i8/16_and_mask', '__not_equal_i8/16_and_mask' functions
x&255 should not be necessary, as long as you are casting to uint8_t and passing as uint8_t parameter. Is it really affects the program behavior?